optimize index updating

The last branch ref that the index was updated to is stored in
.git/annex/index.lck, and the index only updated when the current
branch ref differs.

(The .lck file should later be used for locking too.)

Some more optimization is still needed, since there is some redundancy in
calls to git show-ref.

Locations.hs

@@ -21,6 +21,7 @@ module Locations (
 	gitAnnexJournalDir,
 	gitAnnexJournalLock,
 	gitAnnexIndex,
+	gitAnnexIndexLock,
 	isLinkToAnnex,
 	annexHashes,
 	hashDirMixed,
@@ -136,6 +137,10 @@ gitAnnexJournalLock r = gitAnnexDir r </> "journal.lck"
 gitAnnexIndex :: Git.Repo -> FilePath
 gitAnnexIndex r = gitAnnexDir r </> "index"
 
+{- Lock file for .git/annex/index. -}
+gitAnnexIndexLock :: Git.Repo -> FilePath
+gitAnnexIndexLock r = gitAnnexDir r </> "index.lck"
+
 {- Checks a symlink target to see if it appears to point to annexed content. -}
 isLinkToAnnex :: FilePath -> Bool
 isLinkToAnnex s = ("/.git/" ++ objectDir) `isInfixOf` s