If there are branches in the revset that don't need to be pushed
because they already match the destination, we currently just print
`Nothing changed.` It seems consistent with that to also treat it as
success if there are no branches in the specified set to start
with. This patch makes the command print a warning in that case
instead.
This commit replaces the functions `UserSettings::user_name_placeholder()`` and
`UserSettings::user_email_placeholder()` with `const` `&str`s to emphasize that
the placeholder strings must not be changed to support commits without
names or email addresses made before this change.
