refs: fix creation of reflog entries for symrefs

refs: fix creation of reflog entries for symrefs

The commit 297c09eabb (refs: allow multiple reflog entries for the
same refname, 2024-12-16) added logic to exit early in
`lock_ref_for_update()` after obtaining the required lock. This was
added as a performance optimization on a false assumption that no
further processing was required for reflog-only updates.

However the assumption was wrong.  For a symref's reflog entry, the
update needs to be populated with the old_oid value, but the early
exit skipped this necessary step.

This caused a bug in Git 2.48 in the files backend where target
references of symrefs being updated would create a corrupted reflog
entry for the symref since the old_oid is not populated.

Everything the early exit skipped in the code path is necessary for
both regular and symbolic ref, so eliminate the mistaken
optimization, and also add a test to ensure that such an issue
doesn't arise in the future.

Reported-by: Nika Layzell <nika@thelayzells.com>
Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>