fix(repo): commit page fails to render unsigned commits with a different committer (#39381)

Since #39229 the commit page header dereferences
`.Verification.CommittingUser` when the committer is not the author.
`Verification` is `nil` for unsigned commits (see `repo.Diff`), so
opening such a commit — a rebased or cherry-picked one, for example —
logs a template error and the page comes out truncated:

```
Render failed: failed to render template: repo/commit_page, error: template error: builtin(bindata):repo/commit_page:138:22 : executing "repo/commit_page" at <.Verification.CommittingUser>: nil pointer evaluating interface {}.CommittingUser
```

This guards the access and adds an integration test that creates a
commit with distinct author and committer identities and checks the page
renders completely (the status stays 200 on a mid-render failure, so the
test looks at the body).

_The fix was worked out with help from an AI assistant; I reviewed and
tested it myself._

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/test/utils.go

@@ -64,7 +64,7 @@ func ParseJSONRedirect(buf []byte) (ret struct {
 }
 
 func IsNormalPageCompleted(s string) bool {
-	return strings.Contains(s, `<footer class="page-footer"`) && strings.Contains(s, `</html>`)
+	return strings.Contains(s, `<footer class="page-footer"`) && strings.HasSuffix(strings.TrimSpace(s), `</html>`)
 }
 
 func MockVariableValue[T any](p *T, v ...T) (reset func()) {