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

[lljbash]

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.

gitui/src/popups/commit.rs

Lines 160 to 187 in e08d954

	pub fn show_editor(
	&mut self,
	changes: Vec<StatusItem>,
	) -> Result<()> {
	let file_path = sync::repo_dir(&self.repo.borrow())?
	.join("COMMIT_EDITMSG");
	{
	let mut file = File::create(&file_path)?;
	file.write_fmt(format_args!(
	"{}\n",
	self.input.get_text()
	))?;
	file.write_all(
	strings::commit_editor_msg(&self.key_config)
	.as_bytes(),
	)?;
	file.write_all(b"\n#\n# Changes to be committed:")?;
	for change in changes {
	let status_char =
	Self::item_status_char(change.status);
	let message =
	format!("\n#\t{status_char}: {}", change.path);
	file.write_all(message.as_bytes())?;
	}
	}

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.

[extrawurst]

Still happy to take a PR that adds the diff into the commit template