Added a new feature that started out with me wanting a way to undo a
`git-annex drop`, but turned into something rather more powerful. The `--in`
option can now be told to match files that were in a repository at some
point in the past. For example, `git annex get --in=here@{yesterday}` will
get any files that have been dropped over the past day.

While git-annex's location tracking info is stored in git and thus versioned,
very little of it makes use of past versions of the location tracking info
(only `git annex log`). I'm happy to have finally found a use for it!

OB Windows porting: Fixed a bug in the symlink calculation code.
Sounds simple; took 2 hours!

Also various bug triage; updated git version on OSX; forwarded bug about
DAV-0.6 being broken upstream; fixed a bug with initremote in 
encryption=pubkey mode. Backlog is 65 messages.

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Today's work was sponsored by Brock Spratlen.
