configuration to disable automatic merge conflict resolution

* Added annex.resolvemerge configuration, which can be set to false to
  disable the usual automatic merge conflict resolution done by git-annex
  sync and the assistant.
* sync: Added --no-resolvemerge option.

Note that disabling merge conflict resolution is probably not a good idea
in a direct mode repo or adjusted branch. Since updates to both are done
outside the usual work tree, if it fails the tree is not left in a
conflicted state, and it would be hard to manually resolve the conflict.
Still, made annex.resolvemerge be supported in those cases for consistency.

This commit was sponsored by Riku Voipio.

Assistant/Threads/Merger.hs

@@ -78,6 +78,7 @@ onChange file
 					]
 				void $ liftAnnex $ Command.Sync.merge
 					currbranch Command.Sync.mergeConfig
+					def
 					Git.Branch.AutomaticCommit
 					changedbranch
 	mergecurrent _ = noop