sync: Automatically resolve merge conflict between and annexed file and a regular git file.

This is a new feature, it was not handled before, since it's a bit of an
edge case. However, it can be handled exactly the same as a file/dir
conflict, just leave the non-annexed item alone.

While implementing this, the core resolveMerge' function got a lot simpler
and clearer. Note especially that where before there was an asymetric call to
stagefromdirectmergedir, now graftin is called symmetrically in both cases.

And, in order to add that `graftin us`, the current branch needed to be
known (if there is no current branch, there cannot be a merge conflict).
This led to some cleanups of how autoMergeFrom behaved when there is no
current branch.

This commit was sponsored by Philippe Gauthier.

Assistant/Threads/Merger.hs

@@ -83,7 +83,7 @@ onChange file
 				[ "merging", Git.fromRef changedbranch
 				, "into", Git.fromRef current
 				]
-			void $ liftAnnex  $ autoMergeFrom changedbranch
+			void $ liftAnnex  $ autoMergeFrom changedbranch (Just current)
 	mergecurrent _ = noop
 
 	handleDesynced = case fromTaggedBranch changedbranch of