distributed migration for pull and sync --content

pull, sync: When operating on content, automatically hard link objects
that have been migrated.

Added annex.syncmigrations config that can be set to false to prevent
pull and sync from migrating object content.

I think that true is a good default for this config, because it avoids
users having to re-download migrated content or learning about migration.
But, some users will surely not like it, whether because it does take some
time (especially for the first git-annex branch scan when there is a long
history), or because they want to deal with it manually, or because their
filesystem doesn't support hard links and they don't want it to copy
objects.

Sponsored-by: k0ld on Patreon

Command/Migrate.hs

@@ -59,9 +59,7 @@ seek o
 	| updateOption o || applyOption o = do
 		unless (null (migrateThese o)) $
 			error "Cannot combine --update or --apply with files to migrate."
-		streamNewDistributedMigrations (not (applyOption o)) $ 
-			\oldkey newkey ->
-				commandAction $ update oldkey newkey
+		seekDistributedMigrations (not (applyOption o))
 	| otherwise = do
 		withFilesInGitAnnex ww seeker =<< workTreeItems ww (migrateThese o)
 		commitMigration
@@ -73,6 +71,14 @@ seek o
 		, usesLocationLog = False
 		}
 
+seekDistributedMigrations :: Bool -> CommandSeek
+seekDistributedMigrations incremental =
+	streamNewDistributedMigrations incremental $ \oldkey newkey ->
+		-- Not using commandAction because this is not necessarily
+		-- concurrency safe, and also is unlikely to be sped up
+		-- by multiple jobs.
+		void $ includeCommandAction $ update oldkey newkey
+
 start :: MigrateOptions -> Maybe KeySha -> SeekInput -> RawFilePath -> Key -> CommandStart
 start o ksha si file key = do
 	forced <- Annex.getRead Annex.force