avoid bad commits after interrupted direct mode sync (or merge)

It was possible for a interrupted sync or merge in direct mode to
leave the work tree out of sync with the last recorded commit.
This would result in the next commit seeing files missing from the work
tree, and committing their removal.

Now, a direct mode merge happens not only in a throwaway work tree, but using
a temporary index file, and without any commits or index changes
being made until the real work tree has been updated. If the merge is
interrupted, the work tree may have some updated files, but worst case a
commit will redundantly commit changes that come from the merge.

This commit was sponsored by Tony Cantor.

Git/Index.hs

@@ -30,3 +30,7 @@ override index = do
 
 indexFile :: Repo -> FilePath
 indexFile r = localGitDir r </> "index"
+
+{- Git locks the index by creating this file. -}
+indexFileLock :: Repo -> FilePath
+indexFileLock r = indexFile r ++ ".lock"