ensure outgoing sync pushes always succeed when repo is accessible

The fallback branches pushed to contain the uuid of the pusher, which is
ugly. That's why syncing doesn't normally use this method.

The merger deletes fallback branches after merging them, to contain the
ugliness, and so unused doesn't look at data from these branches.
(The fallback git-annex branch is left behind for now.)

Assistant/Threads/Merger.hs

@@ -15,6 +15,7 @@ import qualified Annex.Branch
 import qualified Git
 import qualified Git.Merge
 import qualified Git.Branch
+import qualified Git.Command as Git
 
 thisThread :: ThreadName
 thisThread = "Merger"
@@ -81,6 +82,11 @@ onAdd g file _
 					, show current
 					]
 				void $ Git.Merge.mergeNonInteractive changedbranch g
+				when ("fallback/" `isInfixOf` (show changedbranch)) $
+					void $ Git.runBool "branch"
+						[ Param "-D"
+						, Param $ show changedbranch
+						] g
 		go _ = noop
 
 equivBranches :: Git.Ref -> Git.Ref -> Bool