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.

debian/changelog

@@ -22,6 +22,8 @@ git-annex (5.20140228) UNRELEASED; urgency=medium
   * metadata: To only set a field when it's not already got a value, use
     -s field?=value
   * Run .git/hooks/pre-commit-annex whenever a commit is made.
+  * sync: Automatically resolve merge conflict between and annexed file
+    and a regular git file.
 
  -- Joey Hess <joeyh@debian.org>  Fri, 28 Feb 2014 14:52:15 -0400