This encapsulates the logic to make sure we have something to commit; which is
to
- auto-stage all files if no files are staged and the SkipNoStagedFilesWarning
config is on
- otherwise, prompt the user whether they want to stage all files
- error out if we don't have any files at all
Of these, the first one was only done when committing with the built-in commit
message panel; there's no reason why it shouldn't also be done when committing
with the editor, or when amending, and now it is.
f561007fb7