Allow ref_branch and target_repo mismatches for known branches

[bbhtt]

Allow ref_branch and target_repo mismatches for known branches

Only verify that "stable" refs don't end up in beta repos and
vice-versa. For any other arbitrary ref branches it is not possible to
tell the intended target repo. So unless the ref_branch matches some
known branches that are already in use and have some significance return
"flat-manager-branch-repo" mismatch error

I assume the target_repo on flathub is fixed and can only be stable, beta or test.

The only other case where "branch" is used in manifest is for extensions, but those aren't checked here, and baseapps are exempt too. I could find only these two in known branches. If the list gets too long, might need to figure out a way to load it better.

I have no way of testing this due to not having access to a flathub like backend, hence draft.

[barthalion]

I ended up going with this: 9c2a70d

Not because yours is worse or wrong, I just didn't notice you opened a PR. The long story short is that only builds targetting the test repo (which isn't even an actual repo, just a construct used by buildbot) are allowed to use branch different than "test". This is an implementation detail of our buildbot config, but we're going to stick with it for now.

[bbhtt]

Sure