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>

git-svnimport.perl

@@ -49,7 +49,7 @@ END
 usage if $opt_h;
 
 my $tag_name = $opt_t || "tags";
-my $trunk_name = $opt_T || "trunk";
+my $trunk_name = defined $opt_T ? $opt_T : "trunk";
 my $branch_name = $opt_b || "branches";
 my $project_name = $opt_P || "";
 $project_name = "/" . $project_name if ($project_name);