log migration trees to git-annex branch

This will allow distributed migration: Start a migration in one clone of
a repo, and then update other clones.

commitMigration is a bit of a bear.. There is some inversion of control
that needs some TMVars. Also streamLogFile's finalizer does not handle
recording the trees, so an interrupt at just the wrong time can cause
migration.log to be emptied but the git-annex branch not updated.

Sponsored-by: Graham Spencer on Patreon

Command/ReKey.hs

@@ -1,6 +1,6 @@
 {- git-annex command
  -
- - Copyright 2012-2016 Joey Hess <id@joeyh.name>
+ - Copyright 2012-2023 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -19,6 +19,7 @@ import Annex.ReplaceFile
 import Logs.Location
 import Annex.InodeSentinal
 import Annex.WorkTree
+import Logs.Migrate
 import Utility.InodeCache
 import qualified Utility.RawFilePath as R
 
@@ -88,7 +89,7 @@ perform file oldkey newkey = do
 			giveup $ decodeBS $ quote qp $ QuotedPath file
 				<> " is not available (use --force to override)"
 		)
-	next $ cleanup file newkey
+	next $ cleanup file newkey $ const noop
 
 {- Make a hard link to the old key content (when supported),
  - to avoid wasting disk space. -}
@@ -127,18 +128,23 @@ linkKey file oldkey newkey = ifM (isJust <$> isAnnexLink file)
 					LinkAnnexNoop -> True
 	)
 
-cleanup :: RawFilePath -> Key -> CommandCleanup
-cleanup file newkey = do
-	ifM (isJust <$> isAnnexLink file)
+cleanup :: RawFilePath -> Key -> (MigrationRecord -> Annex ()) -> CommandCleanup
+cleanup file newkey a = do
+	newkeyrec <- ifM (isJust <$> isAnnexLink file)
 		( do
 			-- Update symlink to use the new key.
-			addSymlink file newkey Nothing
+			sha <- genSymlink file newkey Nothing
+			stageSymlink file sha
+			return (MigrationRecord sha)
 		, do
 			mode <- liftIO $ catchMaybeIO $ fileMode <$> R.getFileStatus file
 			liftIO $ whenM (isJust <$> isPointerFile file) $
 				writePointerFile file newkey mode
-			stagePointerFile file mode =<< hashPointerFile newkey
+			sha <- hashPointerFile newkey
+			stagePointerFile file mode sha
+			return (MigrationRecord sha)
 		)
 	whenM (inAnnex newkey) $
 		logStatus newkey InfoPresent
+	a newkeyrec
 	return True