push: enhance unspecified push default warning

push: enhance unspecified push default warning

When the unset push.default warning message is displayed this may be
the first time many users encounter push.default.

Explain in the warning message in a compact manner what push.default
is and what the change means to the end-user to help the users decide.

Signed-off-by: Greg Jacobson <coder5000@gmail.com>
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Helped-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Helped-by: Marc Branchaud <marcnarc@xiplink.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>