Allow only "Rebase and merge" or "Squash and merge" on GitHub master branch, and sync it on git.ruby-lang.org update hook
- Our pull request merge strategy confuses contributors, as described in [Misc #16093].
- In [Misc #16093], some committers did not like accepting a merge commit.
- Allow using GitHub's "Rebase and merge" (suggested by marcandre (Marc-Andre Lafortune)) and "Squash and merge" (for editing a commit message as needed) to committers
- Disable "Create a merge commit" in GitHub repository settings, to avoid concerns found in [Misc #16093].
- Also prohibit pushes to the master branch which are not coming from a pull request by requiring CI in GitHub repository settings.
- To allow looking up an associated GitHub pull request from git commands, add
git notesto "Rebase and merge"d commits in git.ruby-lang.org post-receive hook. (suggested by nobu (Nobuyoshi Nakada))
- Perform GitHub bidirectional sync on git.ruby-lang.org update hook, and accept a push if it does not conflict with "Rebase and merge" on GitHub.
- Even after implementing this ticket, the Ruby's canonical Git repository will continue to be git.ruby-lang.org as is.
- All committers must push non-pull-request commits to git.ruby-lang.org. Direct pushes to GitHub will continue to be disabled.
- This approach has one drawback;
git pushto git.ruby-lang.org might block longer than now.