Consider using `$GIT_DIR/COMMIT_EDITMSG` in `--verbose` mode

[ivy]

I need to think about this one. $GIT_DIR/COMMIT_EDITMSG appears to get special treatment in editors with additional syntax highlighting. In VS Code, files are highlighted. I'm working around it right now by creating a temporary directory with a COMMIT_EDITMSG file inside of it but it isn't quite the same.

It's… tempting to just drop the file in where git commit --verbose normally works but I'm concerned there may be unexpected side effects.