Handle .patch pull requests of github / gitlab / codeberg

[nicolas-graves]

Hi!

Thanks for b4. I would like to suggest supporting patchsets from github / gitlab / codeberg. They all provide a convenient way to fetch a patchset from a pull request (just add .patch in the url once in the pull request), which isn't far from being parsed by b4 because it isn't that far from a mbox file. In my use case, I must use offline mode, so I can't use the git fetching functionality from b4 pr. In summary :

• b4 am with the right options will find patches (am_msgs=... line in mbox.py) but lser.patches will be empty.
• b4 pr definitely contains the code to simulate mbox from github/gitlab/codeberg patchsets, and it does accept piped externally downloaded files, but it relies on the github API, where we could imagine that it uses a fallback to treat the file as its own complete patchset instead of failing with "Could not find pull request info". Maybe the most straightforward implementation is to add an option there.