This moves the code to call push backends into a library that can be
extended to make matching fetch and push decisions based on the URL it
gets, and which could be changed to have built-in implementations
instead of calling external programs.
Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
