avoid making post-merge-conflict-resolution commit when no conflicts were resolved

sync, merge, assistant: When git merge failed for a reason other than a
conflicted merge, such as a crippled filesystem not allowing particular
characters in filenames, git-annex would make a merge commit that could
omit such files or otherwise be bad. Fixed by aborting the whole merge
process when git merge fails for any reason other than a merge conflict.

Annex/AutoMerge.hs

@@ -77,6 +77,10 @@ autoMergeFrom branch currbranch commitmode = do
  - In direct mode, the work tree is not touched here; files are staged to
  - the index, and written to the gitAnnexMergeDir, for later handling by
  - the direct mode merge code.
+ -
+ - Returns false when there are no merge conflicts to resolve.
+ - A git merge can fail for other reasons, and this allows detecting
+ - such failures.
  -}
 resolveMerge :: Maybe Git.Ref -> Git.Ref -> Annex Bool
 resolveMerge us them = do

Annex/Direct.hs

@@ -173,16 +173,18 @@ mergeDirect startbranch oldref branch resolvemerge commitmode = exclusively $ do
 
 	withIndexFile tmpi $ do
 		merged <- stageMerge d branch commitmode
-		r <- if merged
+		ok <- if merged
 			then return True
 			else resolvemerge
-		mergeDirectCleanup d (fromMaybe Git.Sha.emptyTree oldref)
-		mergeDirectCommit merged startbranch branch commitmode
+		if ok
+			then do
+				mergeDirectCleanup d (fromMaybe Git.Sha.emptyTree oldref)
+				mergeDirectCommit merged startbranch branch commitmode
+				liftIO $ whenM (doesFileExist tmpi) $
+					rename tmpi reali
+			else liftIO $ nukeFile tmpi
 
-		liftIO $ whenM (doesFileExist tmpi) $
-			rename tmpi reali
-
-		return r
+		return ok
   where
 	exclusively = withExclusiveLock gitAnnexMergeLock
 

debian/changelog

@@ -36,6 +36,12 @@ git-annex (5.20150931) UNRELEASED; urgency=medium
   * Avoid unncessary write to the location log when a file is unlocked
     and then added back with unchanged content.
   * S3: Fix support for using https.
+  * sync, merge, assistant: When git merge failed for a reason other
+    than a conflicted merge, such as a crippled filesystem not allowing
+    particular characters in filenames, git-annex would make a merge commit
+    that could omit such files or otherwise be bad. Fixed by aborting the
+    whole merge process when git merge fails for any reason other than a
+    merge conflict.
   * Debian: Add torrent library to build-depends as it's packaged now,
     and stop recommending bittornado | bittorrent.
   * Debian: Remove dependency on transformers library, as it is now