Quick commit in merge request

Description

• Make a quick commit when reviewing a merge request.
• Useful for correcting small typos, whitespace, and enforcing conventions. So instead of pinging the author, you can just make the change yourself as a code reviewer.
• Instead of editing the file in the web file editor mode, just highlight a line when you are reviewing the merge request diff, click an inline UI to bring a small text editor box, and make the correction.
• It is limited to a single line. You can only make changes to a single line.
• You cannot create any new lines.
• You cannot delete any lines. (You can delete all the characters in that line though.)
• The inline editor has another box to put a commit message. There is a pre-filled message that says Fix typo.
• When you submit the change, it creates a new commit to the merge request source branch and appears as a new push (version) to the merge request. It is equivalent to as if you made a new commit in the existing web file editor. This is just a UX optimization. So this will kick off any downstream effects, such as pipelines if they are configured, and reset approvals if they are configured.
• Same permissions. If you have permissions to commit to the merge request branch (i.e. you can do so by clicking the existing Edit button and submitting a change to the branch), then you can make this change.
• After you submit the change, it should reload the merge request so that you are looking the latest version. (If possible, re-loading without refreshing would be better.)

Out of scope

• Doesn't send any notifications. The person making the change can ping the original merge request author in the merge request letting them know they made a small change.