rename --exec to --receive-pack for push and send-pack

rename --exec to --receive-pack for push and send-pack

For now it's just to get a more descriptive name.  Later we might update the
push protocol to run more than one program on the other end.  Moreover this
matches better the corresponding config option remote.<name>. receivepack.

--exec continues to work

Signed-off-by: Uwe Kleine-König <zeisberg@informatik.uni-freiburg.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>