[Feature] Update existing PR title and body with submit --auto #624

Requested Behaviour

When using gh stack submit --auto, I would like it to update the titles and descriptions of existing PRs, similar to how new PRs are filled out. For example, if the title of the first commit changes from chore: update lib to 0.3 to chore: update lib to 0.3.1, I would want the PR title to also be updated to chore: update lib to 0.3.1.

This would be useful to me since I essentially write the PR title and body in the commit message, as done in Gerrit/email workflows. Currently, the most up to date commit message is hidden away in a tab, with the obvious text being potentially out of date. The feature would allow me to have a single source of truth for the commit-level documentation.

Alternatives

1. I do it manually; no code change needed. I don't want to do it manually.
2. Separate script which checks out commits and updates PRs. This is more overhead on the commit process: PRs being updated on submit seems more natural to me.

As far as I can tell, there is no existing issue for this, open or closed.