gitweb: Add support for per project git URLs

gitweb: Add support for per project git URLs

It is now possible for project to have individual clone/fetch URLs.
They are provided in new file 'cloneurl' added below project's
$GIT_DIR directory.

If there is no cloneurl file, concatenation of git base URLs with
project name is used.

This is merge of Jakub Narebski and David Rientjes
  gitweb: Show project's git URL on summary page
with Aneesh Kumar
  gitweb: Add support for cloneurl.
  gitweb: Support multiple clone urls
patches.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>