Use `git commit --verbose` when opening the external editor on a commit message

**Describe the solution you'd like**
Opening the external editor is actually not done by `git commit` command, so we may need to manually put the diff into the created COMMIT_EDITMSG file.

https://github.com/gitui-org/gitui/blob/e08d954573f2da5d66ace2822f439ee0b97ade34/src/popups/commit.rs#L160-L187

**Describe alternatives you've considered**
Or horner `commit.verbose` configuration variable. (#900)

**Additional context**
It would be useful to have a full diff in context when I edit commit messages in an external editor. This would allow me to benefit from completions and copilot, etc.