fix(pull): sign the commit when updating a branch by merge (#38441) (#38499)

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
Backport #38441

routers/web/repo/issue_view.go

@@ -495,7 +495,7 @@ func (prInfo *pullRequestViewInfo) prepareMergeBoxCommitSigning(ctx *context.Con
 
 	wontSignReason := ""
 	if ctx.Doer != nil {
-		sign, key, _, err := asymkey_service.SignMerge(ctx, pull, ctx.Doer, ctx.Repo.GitRepo)
+		sign, key, _, err := asymkey_service.SignMerge(ctx, pull, ctx.Doer, ctx.Repo.GitRepo, pull.BaseBranch, pull.GetGitHeadRefName())
 		data.willSign = sign
 		data.signingKeyMergeDisplay = asymkey_model.GetDisplaySigningKey(key)
 		if err != nil {