The `repo.SingleRelease` handler was broken by gitea#29149, as the
switch to `getReleaseInfos` stopped returning tags without an associated
release. This resulted in the web UI showing a 404 when trying to view a
tag without a release.
This restores the functionality by explicitly including tags in the
search, and also adds tests to exercise the fix.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>