submit: better handling of divergent HEAD for pushed, but not published branches

When submitting a branch, git-spice attempts to match the branch to open PRs on the repo in case the PR was opened without using git-spice's submit command.
Right now, this will refuse to match branches with the same name that have a different HEAD.

This has led to the following scenario for some users:

1. Use git-spice to submit branch but not publish a PR.
2. Create PR manually
3. A CI job that updates the PR (e.g. adjusting formatting)
4. Make local updates and try to submit to update the branch

The submit fails because the branch matching fails (because of divergent HEAD).
We need the following fixes here:

- when rejecting a branch because of a divergent head, prompt the user in case they know for sure that they own this branch
- submit should probably remember that it pushed this branch previously, so even if there's no PR information, it's owned by us