This adds a builtin "push" command, which is largely just a C'ification of
the "git-push.sh" script.

Now, the reason I did it as a built-in is partly because it's yet another
step on relying less on shell, but it's actually mostly because I've
wanted to be able to push to _multiple_ repositories, and the most obvious
and simplest interface for that would seem be to just have a "remotes"
file that has multiple URL entries.

(For "pull", having multiple entries should either just select the first
one, or you could fall back on the others on failure - your choice).

And quite frankly, it just became too damn messy to do that in shell.
Besides, we actually have a fair amount of infrastructure in C, so it just
wasn't that hard to do.

Of course, this is almost totally untested. It probably doesn't work for
anything but the one trial I threw at it. "Simple" doesn't necessarily
mean "obviously correct".
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>