git-svn: Translate invalid characters in refname

git-svn: Translate invalid characters in refname

In git some characters are invalid as documented
in git-check-ref-format. In subversion these characters might
be valid, so a translation is required.

This patch does this translation by url escaping characters, that
are not allowed.

Credit goes to Eric Wong, martin f. krafft and Jan Hudec

Signed-off-by: Robert Ewald <robewald@gmx.net>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>