git svn: fix fetch where glob is on the top-level URL

In cases where the top-level URL we're tracking is the path we
glob against, we can once again track odd repositories that keep
branches/tags at the top level.  This regression was introduced
in commit 6f5748e14cc5bb0a836b649fb8e2d6a5eb166f1d.

Thanks to Daniel Cordero for the original bug report and
bisection.

Signed-off-by: Eric Wong <normalperson@yhbt.net>