Misc #16094

Updated by k0kubun (Takashi Kokubun) 12 months ago

## Problem 
 * Our pull request merge strategy confuses contributors, as described in [Misc #16093]. 
 * In [Misc #16093], some committers did not like accepting a merge commit. 

 ## Solution 
 * Allow using GitHub's "Rebase and merge" (suggested to committers, as suggested by @marcandre) or "Squash and merge" to committers @marcandre 
   * To be discussed: Which should be the default? "Squash and merge"? 
   * Disable other merge modes like "Create a merge commit" and "Squash and merge" in GitHub repository settings. 
   * Also prohibit pushes to the master branch which are not coming from a pull request by requiring CI in GitHub repository settings. 
 * Perform GitHub bidirectional sync on pre-receive hook, and accept a push if it does not conflict with "Rebase and merge" on GitHub. 

 ## Notes 
 * Even after implementing this ticket, the Ruby's canonical Git repository will continue to be as is. 
 * All committers must push non-pull-request commits to Direct pushes to GitHub will continue to be disabled. 
 * This approach has one drawback; `git push` to might block longer than now.