Closes #38651 

`git_panel.set_amend_pending(false, cx);` was being called before
`git_panel.commit_changes(...)` which was causing the commit buffer to
be cleared/reset before actually sending the commit request to git.

Introduced by #35268 which added clear buffer functionality to the
`set_amend_pending` function.

Release Notes:

- Fix git amend on panel sending "Update ..." instead of the original
commit message
- FIx git amend button not working