migrate: Fix support for --backend option.

Command/Migrate.hs

@@ -27,15 +27,20 @@ seek :: [CommandSeek]
 seek = [withBackendFilesInGit start]
 
 start :: CommandStartBackendFile
-start (_, Nothing) = return Nothing
-start (file, Just newbackend) = isAnnexed file $ \(key, oldbackend) -> do
+start (file, b) = isAnnexed file $ \(key, oldbackend) -> do
 	exists <- inAnnex key
+	newbackend <- choosebackend b
 	if (newbackend /= oldbackend) && exists
 		then do
 			showStart "migrate" file
 			return $ Just $ perform file key newbackend
 		else
 			return Nothing
+	where
+		choosebackend Nothing = do
+			backends <- Backend.list
+			return $ head backends
+		choosebackend (Just backend) = return backend
 
 perform :: FilePath -> Key -> Backend -> CommandPerform
 perform file oldkey newbackend = do

debian/changelog

@@ -4,6 +4,7 @@ git-annex (0.18) UNRELEASED; urgency=low
     after transferring the file to the remote repository.
     (Did not affect ssh remotes.)
   * fsck: Fix bug in moving of corrupted files to .git/annex/bad/
+  * migrate: Fix support for --backend option.
   * Test suite improvements. Current top-level test coverage: 75%
 
  -- Joey Hess <joeyh@debian.org>  Tue, 11 Jan 2011 16:05:25 -0400

test.hs

@@ -328,8 +328,8 @@ test_migrate = "git-annex migrate" ~: TestList [t False, t True]
 		if usegitattributes
 			then do
 				writeFile ".gitattributes" "* annex.backend=SHA1"
-				git_annex "migrate" [sha1annexedfile] @? "migrate to same backend failed"
-				git_annex "migrate" [annexedfile] @? "migrate to different backend failed"
+				git_annex "migrate" ["-q", sha1annexedfile] @? "migrate to same backend failed"
+				git_annex "migrate" ["-q", annexedfile] @? "migrate to different backend failed"
 			else do
 				git_annex "migrate" [sha1annexedfile, "--backend=SHA1"] @? "migrate to same backend failed"
 				git_annex "migrate" [annexedfile, "--backend=SHA1"] @? "migrate to different backend failed"