git-svn: Make branch use correct svn-remote

git-svn: Make branch use correct svn-remote

The 'branch' subcommand incorrectly had the svn-remote to use hardcoded
as 'svn', the default remote name.  This meant that branches derived
from other svn-remotes would try to use the branch and tag configuration
for the 'svn' remote, potentially copying would-be branches to the wrong
place in SVN, into the branch namespace for another project.

Fix this by using the remote name extracted from the svn info for the
specified git ref.  Add a testcase for this behaviour.

[jc: squashed in a fix to test from Michael J Gruber for older svn (1.4)]

Signed-off-by: Deskin Miller <deskinm@umich.edu>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>