--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.

CmdLine/GitAnnex/Options.hs

@@ -141,6 +141,7 @@ data KeyOptions
 	| WantUnusedKeys
 	| WantSpecificKey Key
 	| WantIncompleteKeys
+	| WantBranchKeys [Branch]
 
 parseKeyOptions :: Bool -> Parser KeyOptions
 parseKeyOptions allowincomplete = if allowincomplete
@@ -152,6 +153,10 @@ parseKeyOptions allowincomplete = if allowincomplete
 	else base
   where
 	base = parseAllOption
+		<|> WantBranchKeys <$> many (option (str >>= pure . Ref)
+			( long "branch" <> metavar paramRef
+			<> help "operate on files in the specified branch or treeish"
+			))
 		<|> flag' WantUnusedKeys
 			( long "unused" <> short 'U'
 			<> help "operate on files found by last run of git-annex unused"