git-cvsimport: do not fail when CVSROOT is /

For CVS repositories with unusual CVSROOT, git-cvsimport would fail:

    $ git-cvsimport -v -C foo -d :pserver:anon:@cvs.example.com:/ foo
    AuthReply: error 0 : no such repository

This patch ensures that the path is never empty, but at least '/'.
Signed-off-by: NPhilippe Bruhat (BooK) <book@cpan.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-cvsimport.perl

@@ -227,6 +227,7 @@ sub conn {
 				$proxyport = $1;
 			}
 		}
+		$repo ||= '/';
 
 		# if username is not explicit in CVSROOT, then use current user, as cvs would
 		$user=(getlogin() || $ENV{'LOGNAME'} || $ENV{'USER'} || "anonymous") unless $user;