Updating a branch by merge produced an unsigned commit even when merges
are configured to be signed. Update by rebase was unaffected.
`Update()` builds a fake reverse PR to switch head and base, and it has
no `Index`, so `pr.GetGitHeadRefName()` resolves `refs/pull/0/head`.
Since #36186 `SignMerge` looks that ref up in the base repository
instead of the temporary merge repo. That lookup fails. The caller
dropped the error, so `sign` stayed false.
Sync fork goes through the same fake-PR path.
Pass both sides of the merge to `SignMerge` as refs and evaluate them in
the temp repo, where `base` and `tracking` always exist.
Tests cover a signed and an unsigned update by merge.
Fixes#38066