Support the git feature `--allow-empty-message`

[ilyvion]

GitHub's gists are actually repos where each revision is a commit without any message. In order to replicate this if you're manually editing a gist repo is to use --allow-empty-message for your commits. But as far as I can see, Fork does not let me make a commit without a message; the commit button is grayed out: and there's no obvious "allow empty message" override anywhere, despite it being something that is supported by Git.

[DanPristupov]

You can add this using a custom command.