Add a config option for remotes to specify a foreign vcs

Add a config option for remotes to specify a foreign vcs

If this is set, the url is not required, and the transport always uses
a helper named "git-remote-<value>".

It is a separate configuration option in order to allow a sensible
configuration for foreign systems which either have no meaningful urls
for repositories or which require urls that do not specify the system
used by the repository at that location. However, this only affects
how the name of the helper is determined, not anything about the
interaction with the helper, and the contruction is such that, if the
foreign scm does happen to use a co-named url method, a url with that
method may be used directly.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>