Display warning for default git push with no push.default config
If a git push without any refspecs is attempted, display a warning.
The current default behavior is to push all matching refspecs, which
may come as a surprise to new users, so the warning shows how
push.default can be configured and what the possible values are.
Traditionalists who wish to keep the current behaviour are also told
how to configure this once and never see the warning again.
Signed-off-by: Finn Arne Gangstad <finnag@pvv.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>