Re: [PATCH 1/2] fsync: add writeout-only mode for fsyncing repo data

Context for reviewers: The mechanism part of this patch is equivalent to [1]
and [2]. With the following changes:

* The configuration code is a bit different and we now expose a
  `writeout-only` mode to the user. This mode is the default on macOS
  to prevent a change in end-user behavior.

* git_fsync now contains the EINTR retry loop internally.

[1] https://lore.kernel.org/git/e1747ce00af7ab3170a69955b07d995d5321d6f3.1637020263.git.gitgitgadget@gmail.com/ (local)
[2] https://lore.kernel.org/git/546ad9c82e8e0c2eb4683f9f360d8f30e2136020.1630108177.git.gitgitgadget@gmail.com/ (local)