Sanity-bot thows branch validation errors for tqtc private LTS branches in public changes

When a change is uploaded to a public repo with a Pick-to: target of a tqtc/private LTS release branch, the bot gives a -2 on the change because it cannot validate that the pick target exists.

This is because tqtc private LTS release branches only exist in tqtc shadow repositories, so validation fails.

The validation check should be bypassed for release branches of known LTS branches. If the branch is actually invalid, the cherry-pick bot will refuse to pick when it reaches the invalid target and alert the patch author/owner by posting a message to the parent gerrit change.