git-svn: find-rev and rebase for SVN::Mirror repositories

git-svn: find-rev and rebase for SVN::Mirror repositories

find-rev and rebase error out on svm because git-svn doesn't trace the
original svn revision numbers back to git commits. The updated test
case, included in the patch, shows the issue and passes with the rest of
the patch applied.

This fixes Git::SVN::find_by_url to find branches based on the
svm:source URL, where useSvmProps is set. Also makes sure cmd_find_rev
and working_head_info use the information they have to correctly track
the source repository. This is enough to get find-rev and rebase
working.

Signed-off-by: João Abecasis <joao@abecasis.name>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>