contrib/git-svn: strip 'git-svn-id:' when commiting to SVN

contrib/git-svn: strip 'git-svn-id:' when commiting to SVN

We regenerate and use git-svn-id: whenever we fetch or otherwise
commit to remotes/git-svn.  We don't actually know what revision
number we'll commit to SVN at commit time, so this is useless.
It won't throw off things like 'rebuild', though, which knows to
only use the last instance of git-svn-id: in a log message

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>