git-svn: avoid crashing svnserve when creating new directories

git-svn: avoid crashing svnserve when creating new directories

When sorting directory names by depth (slash ("/") count) and
closing the deepest directories first (as the protocol
requires), we failed to put the root baton (with an empty string
as its key "") after top-level directories (which did not have
any slashes).

This resulted in svnserve being in a situation it couldn't
handle and caused a segmentation fault on the remote server.

This bug did not affect users of DAV and filesystem repositories.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Confirmed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <junkio@cox.net>