commit-messages.md: make our git commit message style guide public

So we can link open source contributors to it.

Updates #cleanup

Change-Id: I02f612b38db9594f19b3be5d982f58c136120e9a
Co-authored-by: James Sanderson <jsanderson@tailscale.com>
Co-authored-by: Will Norris <will@tailscale.com>
Co-authored-by: James Tucker <james@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f