Left a few Prelude.head's in where it was checked not null and too hard to
remove, etc.

A few were too hard to get rid of, and safe since the code
does check for an empty line.

Safe versions of init and last

It's possible that a ref name might contain a space, this properly
preserves the space.

(although it was used safely)

A crash on parsing was fixed a while ago. This adds support for fully
correctly parsing multiline git config values, using git config --null.

Since git-annex-shell configlist uses normal git config output, I left in
support for that too; the two forms of config output can be easily
identified by the parser. Since configlist only prints the annex.uuid
config, there's no risk of multiline values there, so no need to change it.

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Constructors and configuration make sense in separate modules.
A separate Git.Types is needed to avoid cycles.

Could have just used hGetContentsStrict here, but that would require
storing all the shas in memory. Since this is called at the end of a
git-annex run, it may have created a *lot* of shas, so I avoid that memory
use and stream them out like before.

avoids a redundant call to git show-ref

avoids a useless diff from git-annex..refs/heads/git-annex

avoids reading the config of a local remote twice in a row

Testing b9ac5854, it didn't find the
optimal union merge, the second sha was the one to use, at least in
the case I tried. Let's just try all shas to see if any can be reused.

I stopped using the expensive nub, so despite the use of sets to
sort/uniq file contents, this is probably as fast or faster than it
was before.