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

Logs.hs

@@ -1,6 +1,6 @@
 {- git-annex log file names
  -
- - Copyright 2013-2021 Joey Hess <id@joeyh.name>
+ - Copyright 2013-2023 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -156,6 +156,11 @@ exportLog = "export.log"
 exportTreeGraftPoint :: RawFilePath
 exportTreeGraftPoint = "export.tree"
 
+{- This is not a log file, it's where migration treeishes get grafted into
+ - the git-annex branch. -}
+migrationTreeGraftPoint :: RawFilePath
+migrationTreeGraftPoint = "migrate.tree"
+
 {- The pathname of the location log file for a given key. -}
 locationLogFile :: GitConfig -> Key -> RawFilePath
 locationLogFile config key =