remote.c has a global set of URL rewrites, accessed by alias_url and
make_rewrite. Wrap them in a new "struct rewrites", passed to alias_url
and make_rewrite. This allows adding other sets of rewrites.
Signed-off-by: Josh Triplett <josh@joshtriplett.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>