contrib/git-svn: create a more recent master if one does not exist

contrib/git-svn: create a more recent master if one does not exist

In a new repository, the initial fetch creates a master branch
if one does not exist so HEAD has something to point to.

It now creates a master at the end of the initial fetch run,
pointing to the latest revision.  Previously it pointed to the
first revision imported, which is generally less useful.

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