Add `gitcommit` and `gitrebase` language?

## Language name

- Git Commit
- Git Rebase

## URL of example repository

I think this is N/A, since a normal repository wouldn't commit any files that are in the `.git` directory.

## URL of syntax highlighting grammar

- https://github.com/textmate/git.tmbundle/blob/master/Syntaxes/Git%20Commit%20Message.tmLanguage
- https://github.com/textmate/git.tmbundle/blob/master/Syntaxes/Git%20Rebase%20Message.tmLanguage

## Most popular extensions

Likely N/A, but the filenames are

- `COMMIT_EDITMSG`
- `git-rebase-todo`

## Detected language

None/Plain text

***

While the files themselves are unlikely to be present in a repository (there are [a lot of results for `COMMIT_EDITMSG`](https://github.com/search?q=filename%3ACOMMIT_EDITMSG&type=Code), though), GitHub also uses linguist to highlight code blocks in Markdown, per my understanding.

Considering that these are common git use cases, I believe that the option to highlight their syntax in Markdown should be available. I think that ***Git***Hub should support syntax highlighting for git filetypes :laughing:

If it is reasonable to support syntax highlighting for these types of files, could you please consider it?