proxy-command support for git://

proxy-command support for git://

Here is an updated patch that first looks for GIT_PROXY_COMMAND
in the environment and then git.proxycommand in the repository's
configuration file.  I have left the calling convention the same
argv[1] is the host and argv[2] is the port.

I've taken the hostname parsing verbatim from git_tcp_connect(),
so it should now support an explicit port number and whatever
that business with the square brackets is.  (Should I move this
to a helper function?)

Regarding internal vs. external hosts, the proxy command can
simply run netcat locally to internal hosts, so perhaps that is
sufficient.

Signed-off-by: Junio C Hamano <junkio@cox.net>