Optimize both pre-commit and lock subcommands.

isLocked was doing the expensive check before the cheap one. Let's not
fork git diff twice per file when committing, especially.

git diff is still run more than strictly necessary (ie, more than once)
if multiple unlocked files are being committed. But much better now.

Command/PreCommit.hs

@@ -22,7 +22,7 @@ import qualified Command.Add
 start :: SubCmdStartString
 start file = do
 	-- If a file is unlocked for edit, add its new content to the
-	-- annex, -}
+	-- annex. -}
 	locked <- Command.Lock.isLocked file
 	when (not locked) $ do
 		pairs <- Backend.chooseBackends [file]
@@ -30,7 +30,8 @@ start file = do
 		unless (ok) $ do
 			error $ "failed to add " ++ file ++ "; canceling commit"
 		-- git commit will have staged the file's content;
-		-- drop that and stage the symlink
+		-- drop that and run command queued by Add.state to
+		-- stage the symlink
 		g <- Annex.gitRepo
 		liftIO $ Git.run g ["reset", "-q", "--", file]
 		Annex.queueRun