Fix: Fix setting last updated commit when pushing and update when only auth repo updated

[renatav]

Description (e.g. "Related to ...", etc.)

This PR contains 4 simple fixes:

• After running a command which pushes to the auth repo, we update the last validated commit. However, since we only validated commits that are on the default branch, last validated commit should only be updated when pushing to this branch
• Similarly, when traversing a repo's commit history, only traverse the default branch's history, even if a different branch is currently checked out.
• Replaced an outdated check if a repo is a test repo
• Determining from which commit the update of target repos should start was incorrect in cases when all target repositories were updated up to the same auth commit, but the auth repo was updated in the meantime. When some target repositories are excluded from the update, their last validated commits will be different and that case is not affected by this bug.

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Changelog has been updated, as needed (see CHANGELOG.md)

[dgreisen]

praise: I love how much easier the type hints make it to go from zero to understanding the code.

suggestion (nonblocking): would it be insane to add a committish type or a hash type so that the type hints are more useful? right now a lot of strings are passed around, so have to look at code to see what that string is. I'm thinking a very simple class Committish(str): pass that would make it a lot clearer what was being returned/consumed.

[renatav]

That could be useful, but I think it would be better to address that in a separate PR, as we'd want to update all occurrences of commits. I'll create an issue