git-svn: handle changed svn command-line syntax

git-svn: handle changed svn command-line syntax

Previously, if you passed a revision and a path to svn cp, it meant to look
back at that revision and select that path.  New behaviour is to get the
path then go back to the revision (like other commands that accept @REV
or -rREV do).  The more consistent syntax is not supported by the old
tools, so we have to try both in turn.

Signed-off-by: Sam Vilain <sam.vilain@catalyst.net.nz>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>