Added --branch option to copy, drop, fsck, get, metadata, mirror, move, and
whereis commands. This option makes git-annex operate on files that are
included in a specified branch (or other treeish).
The names of the files from the branch that are being operated on are not
displayed yet; only the keys. Displaying the filenames will need changes
to every affected command.
Also, note that --branch can be specified repeatedly. This is not really
documented, but seemed worth supporting, especially since we may later want
the ability to operate on all branches matching a refspec. However, when
operating on two branches that contain the same key, that key will be
operated on twice.
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