update-ref: do set reflog's old_oid

In git 2.48.1, the `git update-ref` subcommand no longer correctly
updates the reflog in some cases. Specifically, it appears that the
`old_oid` field will not be updated when modifying a branch referenced
by another symbolic ref (e.g. HEAD). This doesn't break the `git
reflog` subcommand, but does break references like `HEAD@{1}`, which
appear to read the `old_oid` field:

  git init -b main
  git commit --allow-empty -m "A"
  git commit --allow-empty -m "B"
  git update-ref -m "reason" refs/heads/main HEAD~ HEAD

The `old_oid` field is now empty (all zeroes). This is only the case in
derived reflogs (in this case .git/logs/HEAD). The reflog for
`refs/heads/main` appears to be updated correctly.

This was broken in 297c09e (refs: allow multiple reflog entries for
the same refname, 2024-12-16).

The reason for that was that there was assumed the flow of
`lock_ref_for_update()` for reflog only updates was to capture the lock
only. But this is wrong since this misses the `old_oid` population. As
such this patch is the correct fix.

Reported-by: Nika Layzell <[email protected]>
Acked-by: Karthik Nayak <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

refs/files-backend.c

@@ -2615,9 +2615,6 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 
 	update->backend_data = lock;
 
-	if (update->flags & REF_LOG_ONLY)
-		goto out;
-
 	if (update->type & REF_ISSYMREF) {
 		if (update->flags & REF_NO_DEREF) {
 			/*