gitweb: Use config file for repository description and URLs

gitweb: Use config file for repository description and URLs

Allow to use configuration variable gitweb.description for repository
description if there is no $GIT_DIR/description file, and multivalued
configuration variable gitweb.url for URLs of a project (to clone or
fetch from) if there is no $GIT_DIR/cloneurl file.

While repository description is shown in the projects list page, so it
is better to use file and not config variable for performance, it is I
think better to use gitweb.url for URLs (as it is shown only on
project summary page).

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>