After a false start, I found a fairly non-intrusive way to deal with it.
Although it only handles transfers -- there may be issues with eg
concurrent dropping of the same key, or other operations.

There is no added overhead when -J is not used, other than an added
inAnnex check. When -J is used, it has to maintain and check a small
Set, which should be negligible overhead.

It could output some message saying that the transfer is being done by
another thread. Or it could even display the same progress info for both
files that are being downloaded since they have the same content. But I
opted to keep it simple, since this is rather an edge case, so it just
doesn't say anything about the transfer of the file until the other
thread finishes.

Since the deferred transfer action still runs, actions that do more than
transfer content will still get a chance to do their other work. (An
example of something that needs to do such other work is P2P.Annex,
where the download always needs to receive the content from the peer.)
And, if the first thread fails to complete a transfer, the second thread
can resume it.

But, this unfortunately means that there's a risk of redundant work
being done to transfer a key that just got transferred.
That's not ideal, but should never cause breakage; the same
thing can occur when running two separate git-annex processes.

The get/move/copy/mirror --from commands had extra inAnnex checks added,
inside the download actions. Without those checks, the first thread
downloaded the content, and then the second thread woke up and
downloaded the same content redundantly.

move/copy/mirror --to is left doing redundant uploads for now. It
would need a second checkPresent of the remote inside the upload
to avoid them, which would be expensive. A better way to avoid
redundant work needs to be found..

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

Not tested yet.

The EvilLinker workaround is removed. That got fixed in ghc 8.0.1,
(per https://ghc.haskell.org/trac/ghc/ticket/8596)
which will finally be used by the windows autobuilder now.

I have not deleted the EvilLinker yet (or closed its bugs).

This commit was sponsored by John Peloquin on Patreon.

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This reverts commit ffa81d2d.

Old win32-extras fails to install. I suspect toolchain breakage because
a lot of stuff is failing on the windows autobuilder.

git annex add, git annex lock etc make multiple seek passes,
and each seek pass checked that files existed. That was unncessary
redundant work.

Fixed by adding a new WorkTreeItem type, make seek actions use it,
and check that the files exist when constructing it.

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

Before, there was a window where interrupting an add could result in the
file being moved into the annex, with no symlink yet created.

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

when storing files there, since that collection is created by initremote.
(This seems to work around some brokenness of the box.com webdav server
which was entering a redirect loop.)

Note that the fix makes locationParent return Nothing instead of "."
when there's no parent directory between the path and the top of the webdav
repo.

This commit was sponsored by André Pereira on Patreon.

This commit was sponsored by Denis Dzyubenko on Patreon.

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