Allow git-svnimport to take "" as the trunk directory.
Some repositories started with the trunk in "/" and then moved it to the
standard "trunk/" location.
On these repositories, the correct thing would be to call git-svnimport -T "",
but because of the way the options are handled, it uses the default "trunk"
instead of the given empty string. This patch fixes that behaviour.
Reported by Leandro Lucarella <llucax@gmail.com>.
Signed-off-by: Alberto Bertogli <albertito@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>