slow, stupid, and safe index updating

Always merge the git-annex branch into .git/annex/index before making a
commit from the index.

This ensures that, when the branch has been changed in any way
(by a push being received, or changes pulled directly into it, or
even by the user checking it out, and committing a change), the index
reflects those changes.

This is much too slow; it needs to be optimised to only update the
index when the branch has really changed, not every time.

Also, there is an unhandled race, when a change is made to the branch
right after the index gets updated. I left it in for now because it's
unlikely and I didn't want to complicate things with additional locking
yet.

doc/bugs/git-annex_branch_corruption.mdwn

@@ -39,6 +39,9 @@ corresponds to its index, and if the branch is at a different ref,
 merge it into the index. I am still considering how to do that atomically;
 what if a push comes in while git-annex is updating its index?
 
+> Now git-annex always updates the index with the git-annex branch, which
+> is a slow, but safe way to avoid this problem. [[done]] --[[Joey]]
+
 ---
 
 ## Workaround