Sanity bot falsely claims "Not an upstream commit" on lts- branches

See Support Python 3.11.N (I25b0a568) · Gerrit Code Review (qt-project.org) for an example.

I suspect this is due to the tqtc/lts- prefix. The bot should probably examine tqtc/lts- branches if the child change also has the prefix. Likely, the bot should be aware of optional "tqtc/" pre-prefix before the "lts-" prefix, as the latter may exist in non-tqtc repos.