Don't try and percent-escape existing percent escapes in git-svn URIs

Don't try and percent-escape existing percent escapes in git-svn URIs

git-svn project names are percent-escaped ever since f5530b8
(git-svn: support for funky branch and project names over HTTP(S),
2007-11-11).

Unfortunately this breaks the scenario where the user hands git-svn an
already-escaped URI.  Fix the regexp to skip over what looks like
existing percent escapes, and test this scenario.

Signed-off-by: Kevin Ballard <kevin@sb.org>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>