Better handle additional commits after a PR has been created

[jsf9k]

💡 Summary

Lineage should better handle the case where it has already created a pull request, but more changes are merged into the relevant skeleton.

Motivation and context

Lineage currently just blindly tacks the additional commits onto the existing pull request. This is problematic if:

• The existing pull request's description does not indicate that there are conflicts
• The additional commits create a conflict

Implementation notes

• Lineage should at a minimum update the pull request description if the new commits generate a conflict and the existing description does not indicate that.
• Even if the new commits do not generate a conflict, Lineage should possibly add a blurb to the pull request description to inform the user that additional commits were added.
• Lineage probably needs to warn the user (via the PR description) if it detects that the PR has been manually modified and there are additional commits to be added. In many cases the user would want to delete the branch and let Lineage recreate it from scratch.

Acceptance criteria

How do we know when this work is done?

• Criterion