add tree-sitter-git-rebase

[the-mikedavis]

Adds syntax highlighting for the interactive rebase TODO file:

with highlighting appropriate for the long and short versions of commands (e.g. "fixup" and "f"), and injections for shell with the exec command.

And the cherry on top: injections for commit message highlighting when you're editing commit messages in a rebase:

I was interested in including this on #1338, but I couldn't get the injections to work. It turns out injections seem to not work when the node being injected has an underscore in its name (e.g. rebase_command instead of rebasecommand). I suspect that there's a rogue regex in the tree-sitter code that tries to ignore injections on unnamed nodes (start with an underscore) that's mistakenly checking for any underscore at all. (edit: meant to say that I think this is a bug upstream in tree-sitter, I'll investigate 🕵️) (edit: now I can't reproduce that behavior and it works even with the underscore 🤔)

These captures are a little arbitrary, I'm open to suggestions on all of them.

[pickfire]

Looks good to me. Thank you. \o/