Incomplete bash completion on git commit: --allow-empty-message and --allow-empty

Hi!

I'm wondering why there isn't --allow-empty and --allow-empty-message in completeion list of git commit command. I'm getting only following flags from v2.18.0 on `git commit --`:

--ahead-behind --include --reedit-message= --all --interactive --reset-author --amend --long --reuse-message= --author= --message= --short --branch --no-edit --signoff --cleanup= --no-post-rewrite --squash= --date= --no-verify --status --dry-run --null --template= --edit --only --untracked-files --file= --patch --verbose --fixup= --porcelain --verify --gpg-sign --quiet

Besides, is there any way to allow empty commit message for a repo, e.g. by adding something to `.git/config`? I couldn't find any in docs.