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 <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

gitweb/gitweb.perl

@@ -69,7 +69,7 @@
 
 # list of git base URLs used for URL to where fetch project from,
 # i.e. full URL is "$git_base_url/$project"
-our @git_base_url_list = ("++GITWEB_BASE_URL++");
+our @git_base_url_list = grep { $_ ne '' } ("++GITWEB_BASE_URL++");
 
 # default blob_plain mimetype and default charset for text/plain blob
 our $default_blob_plain_mimetype = 'text/plain';