svnimport: newer libsvn wants us to ask for the root with "", not "/"

In r27729, libsvn introduced an assert which explicitly
forbids searching the tree at "/". Luckily enough, it
still accepts an empty string "" as the starting point.

http://svn.collab.net/viewvc/svn/trunk/subversion/libsvn_ra/ra_loader.c?r1=27653&r2=27729

Tested against libsvn0-1.5.0-4mdv2009.0 (needs the fix),
libsvn0-1.4.6-5mdv2008.1 (works anyway)

Signed-off-by: P. Christeas <p_christ@hol.gr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>