Provide a way to prompt to amend commit message only

[jasonruesch]

There should be a way to prompt when there are no staged or changed files, but the commit message needs to be amended and still conform to the conventions.

git commit --amend

With no files changed or staged, the amend commit command currently displays the message, "No files added to staging! Did you forget to run git add?" and bypasses the prompt, by design per #7.

I contend that there are reasons to prompt just to fix the commit message without file changes.