started migrate --update

This is most of the way there, but not quite working.

The layout of migrate.tree/ needs to be changed to follow this approach.
git log will list all the files in tree order, so the new layout needs
to alternate old and new keys. Can that be done? git may not document
tree order, or may not preserve it here.

Alternatively, change to using git log --format=raw and extract
the tree header from that, then use
git diff --raw $tree:migrate.tree/old $tree:migrate.tree/new
That will be a little more expensive, but only when there are lots of
migrations.

Sponsored-by: Joshua Antonishen on Patreon

Command/Migrate.hs

@@ -28,21 +28,31 @@ cmd = withAnnexOptions [backendOption, annexedMatchingOptions, jsonOptions] $
 
 data MigrateOptions = MigrateOptions
 	{ migrateThese :: CmdParams
+	, updateOption :: Bool
 	, removeSize :: Bool
 	}
 
 optParser :: CmdParamsDesc -> Parser MigrateOptions
 optParser desc = MigrateOptions
 	<$> cmdParams desc
+	<*> switch
+		( long "update"
+		<> help "update for migrations performed elsewhere"
+		)
 	<*> switch
 		( long "remove-size"
 		<> help "remove size field from keys"
 		)
 
 seek :: MigrateOptions -> CommandSeek
-seek o = do
-	withFilesInGitAnnex ww seeker =<< workTreeItems ww (migrateThese o)
-	commitMigration
+seek o
+	| updateOption o = do
+		unless (null (migrateThese o)) $
+			error "Cannot combine --update with files to migrate."
+		commandAction update
+	| otherwise = do
+		withFilesInGitAnnex ww seeker =<< workTreeItems ww (migrateThese o)
+		commitMigration
   where
 	ww = WarnUnmatchLsFiles "migrate"
 	seeker = AnnexedFileSeeker
@@ -131,3 +141,10 @@ perform onlyremovesize o file oldkey oldkeyrec oldbackend newbackend = go =<< ge
 		| removeSize o = alterKey k $ \kd -> kd { keySize = Nothing } 
 		| otherwise = k
 	afile = AssociatedFile (Just file)
+
+update :: CommandStart
+update = starting "migrate" (ActionItemOther Nothing) (SeekInput []) $ do
+	streamNewDistributedMigrations $ \oldkey newkey -> do
+		liftIO $ print ("migrate", oldkey, newkey)
+	next $ return True
+