This is similar to the pusher thread, but a separate thread because git
pushes can be done in parallel with exports, and updating a big export
should not prevent other git pushes going out in the meantime.

The exportThread only runs at most every 30 seconds, since updating an
export is more expensive than pushing. This may need to be tuned.

Added a separate channel for export commits; the committer records a
commit in that channel.

Also, reconnectRemotes records a dummy commit, to make the exporter
thread wake up and make sure all exports are up-to-date. So,
connecting a drive with a directory special remote export will
immediately update it, and getting online will automatically
update S3 and WebDAV exports.

The transfer queue is not involved in exports. Instead, failed
exports are retried much like failed pushes.

This commit was sponsored by Ewen McNeill.

New table needed to look up what filenames are used in the currently
exported tree, for reasons explained in export.mdwn.

Also, added smart constructors for ExportLocation and ExportDirectory to
make sure they contain filepaths with the right direction slashes.

And some code refactoring.

This commit was sponsored by Francois Marier on Patreon.

Went with a separate db per export remote, rather than a single export
database. Mostly because there will probably not be a lot of separate
export remotes, and it might be convenient to be able to delete a given
remote's export database.

This commit was supported by the NSF-funded DataLad project.

This will allow disabling exports for remotes that are not configured to
allow them. Also, exportSupported will be useful for the external
special remote to probe.

This commit was supported by the NSF-funded DataLad project

Incremental export updates work now too.

This commit was sponsored by Anthony DeRobertis on Patreon.

https://github.com/haskell/cabal/issues/4655

This means that when a module is conditionally imported via ifdef
depending on the OS or build flags, the cabal file has to mirror the
same logic there to only list the module then.

Since there are lots of OS's and lots of combinations of build flags
here, it's rather difficult to know if the cabal file has been completelty
correctly updated to match the source code.

So I am very unhappy with needing to update things in two places. I've
only tested this on linux with most build flags enables; this will
probably need significant time and testing to catch every cabal file
tweak that this change to Cabal requires. And it will be a continual
source of compile failures going forward when the code is modified and
the cabal file not also updated.

DRY DRY DRY, I repeat myself, but: DRY! Sigh..

(Also, had to remove all Build.* that are standalone programs from the
Other-Modules list, because since cabal passes those modules to ghc when
building git-annex, it complains that they use module Main. Those
modules are only used when building with the Makefile anyway, so this
change shouldn't break anything.)

This commit was sponsored by Thomas Hochstein on Patreon.

Security fix: Disallow hostname starting with a dash, which would get
passed to ssh and be treated an option. This could be used by an attacker
who provides a crafted ssh url (for eg a git remote) to execute arbitrary
code via ssh -oProxyCommand.

No CVE has yet been assigned for this hole.
The same class of security hole recently affected git itself,
CVE-2017-1000117.

Method: Identified all places where ssh is run, by git grep '"ssh"'
Converted them all to use a SshHost, if they did not already, for
specifying the hostname.

SshHost was made a data type with a smart constructor, which rejects
hostnames starting with '-'.

Note that git-annex already contains extensive use of Utility.SafeCommand,
which fixes a similar class of problem where a filename starting with a
dash gets passed to a program which treats it as an option.

This commit was sponsored by Jochen Bartl on Patreon.

Added remote configuration settings annex-ignore-command and
annex-sync-command, which are dynamic equivilants of the annex-ignore
and annex-sync configurations.

For this I needed a new DynamicConfig infrastructure. Its implementation
should be as fast as before when there is no dynamic config, and it caches
so shell commands are only run once.

Note that annex-ignore-command exits nonzero when the remote should be ignored.
While that may seem backwards, it allows using the same command for it as
for annex-sync-command when you want to disable both.

This commit was sponsored by Trenton Cronholm on Patreon.

Can be used to override the default timestamps used in log files in the
git-annex branch. This is a dangerous environment variable; use with
caution.

Note that this only affects writing to the logs on the git-annex branch.
It is not used for metadata in git commits (other env vars can be set for
that).

There are many other places where timestamps are still used, that don't
get committed to git, but do touch disk. Including regular timestamps
of files, and timestamps embedded in some files in .git/annex/, including
the last fsck timestamp and timestamps in transfer log files.

A good way to find such things in git-annex is to get for getPOSIXTime and
getCurrentTime, although some of the results are of course false positives
that never hit disk (unless git-annex gets swapped out..)

So this commit does NOT necessarily make git-annex comply with some HIPPA
privacy regulations; it's up to the user to determine if they can use it in
a way compliant with such regulations.

Benchmarking: It takes 0.00114 milliseconds to call getEnv
"GIT_ANNEX_VECTOR_CLOCK" when that env var is not set. So, 100 thousand log
files can be written with an added overhead of only 0.114 seconds. That
should be by far swamped by the actual overhead of writing the log files
and making the commit containing them.

This commit was supported by the NSF-funded DataLad project.

Removed dependency on MissingH, instead depending on the split
library.

After laying groundwork for this since 2015, it
was mostly straightforward. Added Utility.Tuple and
Utility.Split. Eyeballed System.Path.WildMatch while implementing
the same thing.

Since MissingH's progress meter display was being used, I re-implemented
my own. Bonus: Now progress is displayed for transfers of files of
unknown size.

This commit was sponsored by Shane-o on Patreon.

Cryptonite is faster and allocates less, and I want to get rid of
MissingH use.

Note that the new dependency on memory is free; it's a dependency of
cryptonite.

This commit was supported by the NSF-funded DataLad project.

Fixes a build failure of stack build --profile.
See https://ghc.haskell.org/trac/ghc/ticket/8443

This commit was sponsored by Anthony DeRobertis on Patreon.

This commit was supported by the NSF-funded DataLad project.

They are handled close the same as they are by git. However, unlike git,
git-annex sometimes needs to pass the -n parameter when using these.

So, this has the potential for breaking some setup, and perhaps there ought
to be a ANNEX_USE_GIT_SSH=1 needed to use these. But I'd rather avoid that
if possible, so let's see if anyone complains.

Almost all places where "ssh" was run have been changed to support the env
vars. Anything still calling sshOptions does not support them. In
particular, rsync special remotes don't. Seems that annex-rsync-transport
already gives sufficient control there.

(Fixed in passing: Remote.Helper.Ssh.toRepo used to extract
remoteAnnexSshOptions and pass them to sshOptions, which was redundant
since sshOptions also extracts those.)

This commit was sponsored by Jeff Goeke-Smith on Patreon.

This reverts commit fc3925a1.

Need it in stable w/o backports for the ancient autobuilder.

network-uri is available in Debian stable (backports) and testing,
so no need to complicate the cabal file anymore

No changes from 6.20170228; a new version number was needed due to a problem with Hackage.

Building with that library made git-annex not support SHA3; it's time for
that to always be supported in case SHA2 dominoes.

The p2p code made it always be needed.

This commit was sponsored by Anthony DeRobertis on Patreon.

sync: When syncing with a local repository located on a crippled
filesystem, run the post-receive hook there, since it wouldn't get run
otherwise. This makes pushing to repos on FAT-formatted removable drives
update them when receive.denyCurrentBranch=updateInstead.

Made Remote.Git export onLocal, which was cleaned up to not have so many
caveats about its use.

This commit was sponsored by Jeff Goeke-Smith on Patreon.

* Added post-recieve hook, which makes updateInstead work with direct
  mode and adjusted branches.
* init: Set up the post-receive hook.

This commit was sponsored by Fernando Jimenez on Patreon.