assistant: Merge changes from refs/remotes/foo/master into master.

Previously, only sync branches were merged. This makes regular git push
into a repository watched by the assistant auto-merge.

While this does hardcode an assumption about what the remote tracking
branch is named, which some unusual git configurations won't match,
git-annex sync already made the same assumption.

Also, changed behavior when a tracking branch like
refs/remotes/synced/not/master is received. When on the master branch,
that used to get merged into it, but it's the tracking branch for
not/master, so should only be merged in when on the not/master branch.

This commit was sponsored by Ewen McNeill.

Assistant/Threads/Merger.hs

@@ -1,6 +1,6 @@
 {- git-annex assistant git merge thread
  -
- - Copyright 2012 Joey Hess <id@joeyh.name>
+ - Copyright 2012-2017 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -15,6 +15,7 @@ import Utility.DirWatcher.Types
 import qualified Annex.Branch
 import qualified Git
 import qualified Git.Branch
+import qualified Git.Ref
 import qualified Command.Sync
 
 {- This thread watches for changes to .git/refs/, and handles incoming
@@ -63,19 +64,14 @@ onChange file
 		diverged <- liftAnnex Annex.Branch.forceUpdate
 		when diverged $
 			queueDeferredDownloads "retrying deferred download" Later
-	-- Merge only from /synced/ branches, which are pushed by git-annex
-	-- sync and by remote instances of the assistant.
-	-- It would be nice to merge other remote tracking branches,
-	-- but it's hard to get an efficient list of them (git remote -r)
-	| "/synced/" `isInfixOf` file = mergecurrent
-	| otherwise = noop
+	| otherwise = mergecurrent
   where
 	changedbranch = fileToBranch file
 
 	mergecurrent =
 		mergecurrent' =<< liftAnnex (join Command.Sync.getCurrBranch)
 	mergecurrent' currbranch@(Just b, _)
-		| equivBranches changedbranch b =
+		| changedbranch `isRelatedTo` b =
 			whenM (liftAnnex $ inRepo $ Git.Branch.changed b changedbranch) $ do
 				debug
 					[ "merging", Git.fromRef changedbranch
@@ -88,10 +84,17 @@ onChange file
 					changedbranch
 	mergecurrent' _ = noop
 
-equivBranches :: Git.Ref -> Git.Ref -> Bool
-equivBranches x y = base x == base y
+{- Is the first branch a synced branch or remote tracking branch related
+ - to the second branch, which should be merged into it? -}
+isRelatedTo :: Git.Ref -> Git.Ref -> Bool
+isRelatedTo x y
+	| basex /= takeDirectory basex ++ "/" ++ basey = False
+	| "/synced/" `isInfixOf` Git.fromRef x = True
+	| "refs/remotes/" `isPrefixOf` Git.fromRef x = True
+	| otherwise = False
   where
-	base = takeFileName . Git.fromRef
+	basex = Git.fromRef $ Git.Ref.base x
+	basey = Git.fromRef $ Git.Ref.base y
 
 isAnnexBranch :: FilePath -> Bool
 isAnnexBranch f = n `isSuffixOf` f