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

Annex/Ingest.hs

@@ -19,6 +19,7 @@ module Annex.Ingest (
 	finishIngestUnlocked,
 	cleanOldKeys,
 	addSymlink,
+	genSymlink,
 	makeLink,
 	addUnlocked,
 	CheckGitIgnore(..),
@@ -38,6 +39,7 @@ import Annex.MetaData
 import Annex.CurrentBranch
 import Annex.CheckIgnore
 import Logs.Location
+import qualified Git
 import qualified Annex
 import qualified Database.Keys
 import Config
@@ -320,9 +322,12 @@ makeLink file key mcache = flip catchNonAsync (restoreFile file key) $ do
 
 {- Creates the symlink to the annexed content, and stages it in git. -}
 addSymlink :: RawFilePath -> Key -> Maybe InodeCache -> Annex ()
-addSymlink file key mcache = do
+addSymlink file key mcache = stageSymlink file =<< genSymlink file key mcache
+
+genSymlink :: RawFilePath -> Key -> Maybe InodeCache -> Annex Git.Sha
+genSymlink file key mcache = do
 	linktarget <- makeLink file key mcache
-	stageSymlink file =<< hashSymlink linktarget
+	hashSymlink linktarget
 
 {- Parameters to pass to git add, forcing addition of ignored files.
  -

Annex/Locations.hs

@@ -54,6 +54,8 @@ module Annex.Locations (
 	gitAnnexRestageLock,
 	gitAnnexAdjustedBranchUpdateLog,
 	gitAnnexAdjustedBranchUpdateLock,
+	gitAnnexMigrateLog,
+	gitAnnexMigrateLock,
 	gitAnnexMoveLog,
 	gitAnnexMoveLock,
 	gitAnnexExportDir,
@@ -407,6 +409,13 @@ gitAnnexAdjustedBranchUpdateLog r = gitAnnexDir r P.</> "adjust.log"
 gitAnnexAdjustedBranchUpdateLock :: Git.Repo -> RawFilePath
 gitAnnexAdjustedBranchUpdateLock r = gitAnnexDir r P.</> "adjust.lck"
 
+{- .git/annex/migrate.log is used to log migrations before committing them. -}
+gitAnnexMigrateLog :: Git.Repo -> RawFilePath
+gitAnnexMigrateLog r = gitAnnexDir r P.</> "migrate.log"
+
+gitAnnexMigrateLock :: Git.Repo -> RawFilePath
+gitAnnexMigrateLock r = gitAnnexDir r P.</> "migrate.lck"
+
 {- .git/annex/move.log is used to log moves that are in progress,
  - to better support resuming an interrupted move. -}
 gitAnnexMoveLog :: Git.Repo -> RawFilePath