[ruby-core:94225] [Ruby master Misc#16093] Disable changing the first parent instead of prohibiting a merge commit

Issue #16093 has been reported by k0kubun (Takashi Kokubun).

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Misc #16093: Disable changing the first parent instead of prohibiting a merge commit
https://bugs.ruby-lang.org/issues/16093

* Author: k0kubun (Takashi Kokubun)
* Status: Open
* Priority: Normal
* Assignee: 
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## Background
* When we migrated the canonical Ruby repository from Subversion to Git [Misc #14632], in that ticket nobody had objected to allowing a merge commit in the repository.
* However, @mame has [prohibited](https://github.com/ruby/ruby-commit-hook/commit/d7759cca6282741ecc9c46053166a1f5f5779c10#diff-7e61afe505452158c45dfa32ea7d7a14) to push a merge commit to the master branch.
  * His motivation was to make it possible to efficiently list up revisions to be built by his [rubyfarmer](https://github.com/mame/rubyfarmer) without having a data store.
* If we do not make a merge commit, there's only one way to make a pull request "Merged" on GitHub ruby/ruby:
  * Push commits in the pull requests to the master branch when their parent revision is the same as the master branch's one.
* Therefore, we have not been able to make a pull request "Merged" when a pull request's branch needs to be rebased before pushing it to the master branch.
  * But force-pushing the rebased commits to their author's branch is a bad habit and we do not want to do so.

## Problem
* Some contributors get confused when their pull request is committed to the master branch but it's marked as "Closed" on GitHub [Misc #16089].
* Even though we clarified the situation in https://bugs.ruby-lang.org/projects/ruby/wiki/HowToContribute, a first-time contributor could be confused and the person might complain about it to the committer.

## Solution
1. Improve the rubyfarmer's implementation to make it work even if we had merge commits. https://github.com/mame/rubyfarmer/pull/1
2. To maintain a consistent linear history in the git log even after allowing merge commits, implement a guard to prevent a ["foxtrot merge"](https://blog.developer.atlassian.com/stop-foxtrots-now/) in the pre-receive hook.
  * Details: https://devblog.nestoria.com/post/98892582763/maintaining-a-consistent-linear-history-for-git
3. Allow pushing merge commits to the master branch.

I'll implement the 2 and 3 soon, assuming that @mame had been the only person who had an objection to merge commits. Please let me know if you have any opinion about this.



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