gitweb: Check git base URLs before generating URL from it

gitweb: Check git base URLs before generating URL from it

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: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>