--branch, stage 1

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.

Command/Sync.hs

@@ -441,7 +441,7 @@ seekSyncContent o rs = do
 		Just WantAllKeys -> Just <$> genBloomFilter (seekworktree mvar [])
 		_ -> seekworktree mvar [] (const noop) >> pure Nothing
 	withKeyOptions' (keyOptions o) False
-		(seekkeys mvar bloom) 
+		(return (seekkeys mvar bloom))
 		(const noop)
 		[]
 	finishCommandActions