Closes #25976

There's a couple states that this covers:
- upon `git init`, no footer is shown at all
- after 1 commit (or when on any parentless commit), the uncommit button
is ~disabled~ hidden
- otherwise commit button is shown

Also updated the button with "meta" tooltip showing human readable
description and git command.

Release Notes:

- N/A

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Co-authored-by: Nate Butler <iamnbutler@gmail.com>