Check if each of git base URLs in @git_base_url_list is true before
appending "/$project" to it to generate project URL.

This fixes the error that for default configuration for gitweb in
Makefile, with GITWEB_BASE_URL empty (and "++GITWEB_BASE_URL++" being
"" in gitweb.cgi), we had URL of "/$project" in the summary view.
Signed-off-by: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <junkio@cox.net>