refs: fix migration of reflogs respecting "core.logAllRefUpdates"

In 246cebe (refs: add support for migrating reflogs, 2024-12-16) we
have added support to git-refs(1) to migrate reflogs between reference
backends. It was reported [1] though that not we don't migrate reflogs
for a subset of references, most importantly "refs/stash".

This issue is caused by us still honoring "core.logAllRefUpdates" when
trying to migrate reflogs: we do queue the updates, but depending on the
value of that config we may decide to just skip writing the reflog entry
altogether. And given that:

  - The default for "core.logAllRefUpdates" is to only create reflogs
    for branches, remotes, note refs and "HEAD"

  - "refs/stash" is neither of these ref types.

We end up skipping the reflog creation for that particular reference.

Fix the bug by setting `REF_FORCE_CREATE_REFLOG`, which instructs the
ref backends to create the reflog entry regardless of the config or any
preexisting state.

[1]: <Z5BTQRlsOj1sygun@tapette.crustytoothpaste.net>

Reported-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

refs.c

@@ -1282,7 +1282,7 @@ int ref_transaction_update_reflog(struct ref_transaction *transaction,
 
 	assert(err);
 
-	flags |= REF_LOG_ONLY | REF_NO_DEREF;
+	flags |= REF_LOG_ONLY | REF_FORCE_CREATE_REFLOG | REF_NO_DEREF;
 
 	if (!transaction_refname_valid(refname, new_oid, flags, err))
 		return -1;

t/t1460-refs-migrate.sh

@@ -224,6 +224,23 @@ do
 			test_commit --date "100003000 +0700" --no-tag -C repo second &&
 			test_migration repo "$to_format"
 		'
+
+		test_expect_success "$from_format -> $to_format: stash is retained" '
+			test_when_finished "rm -rf repo" &&
+			git init --ref-format=$from_format repo &&
+			(
+				cd repo &&
+				test_commit initial A &&
+				echo foo >A &&
+				git stash push &&
+				echo bar >A &&
+				git stash push &&
+				git stash list >expect.reflog &&
+				test_migration . "$to_format" &&
+				git stash list >actual.reflog &&
+				test_cmp expect.reflog actual.reflog
+			)
+		'
 	done
 done