git-svn: fix svn.pushmergeinfo handling of svn+ssh usernames.

git-svn: fix svn.pushmergeinfo handling of svn+ssh usernames.

Previously, svn dcommit of a merge with svn.pushmergeinfo set would
get error messages like "merge parent <X> for <Y> is on branch
svn+ssh://gcc.gnu.org/svn/gcc/trunk, which is not under the git-svn root
svn+ssh://jason@gcc.gnu.org/svn/gcc!"

So, let's call remove_username (as we do for svn info) before comparing
rooturl to branchurl.

Signed-off-by: Jason Merrill <jason@redhat.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>