better limiting of start actions to only run whenAnnexed

Mostly only refactoring, but this does remove one redundant stat of the
symlink by copy.

Command/Unlock.hs

@@ -22,12 +22,12 @@ def =
 		c n = command n paramPaths seek
 
 seek :: [CommandSeek]
-seek = [withFilesInGit start]
+seek = [withFilesInGit $ whenAnnexed start]
 
 {- The unlock subcommand replaces the symlink with a copy of the file's
  - content. -}
-start :: FilePath -> CommandStart
-start file = isAnnexed file $ \(key, _) -> do
+start :: FilePath -> (Key, Backend Annex) -> CommandStart
+start file (key, _) = do
 	showStart "unlock" file
 	next $ perform file key