Add support for pushing stacked reviews to GitHub, GitLab, etc.

[martinvonz]

Description

For users who prefer one review per commit (as we do in this project), it would be useful to have some support for pushing a stack of commits and have it automatically create (or reuse) PRs for each commit. In the case of GitHub, we should also set the base branch appropriately then. I don't know about GitLab or any other providers we would care about.

Maybe we'd want commands like jj github push and jj gitlab push or something like, to make it clear that they're specifically for those providers.

[beatrixada]

I love this feature in Sapling and would love to see it in Jujutsu as well!

[FirelightFlagboy]

git town propose have a similar feature where it would create a new pull request for the current branch and setting the base as the configured parent branch.

[steveklabnik]

See also https://github.com/ejoffe/spr

and @sunshowers's fork, which has experimental jj support https://github.com/sunshowers/spr

[sunshowers]

Note that spr (and all other stacked diff tooling for GH, afaik) requires that you have push access to the repo you're proposing changes to.