A common complaint is that `git annex fsck` in a bare repository complains
about missing content of deleted files. That's because in a bare
repository, git-annex operates on all versions of all files. Today I added
a --branch option, so if you only want to check say, the master branch, you
can: `git annex fsck --branch master`

The new option has other uses too. Want to get all the files in the v1.0
tag? `git annex get --branch v1.0`

It might be worth revisiting the implicit --all behavior for bare
repositories. It could instead default to --branch HEAD or something like
that. But I'd only want to change that if there was a strong consensus in
favor.

Over 3/4th of the time spent implementing --branch was spent in
adjusting the output of commands, to show "branch:file" is being
operated on. How annoying.
