git-instaweb: Configure it to work with new gitweb structure

git-instaweb: Configure it to work with new gitweb structure

git-instaweb in its current form (re)creates gitweb.cgi and
(some of) required static files in $GIT_DIR/gitweb/ directory.
Splitting gitweb would make it difficult for git-instaweb to
continue with this method.

Use the instaweb.gitwebdir config variable to point git-instaweb script
to a global directory which contains gitweb files as server root
and the httpd.conf along with server logs and pid go into
'$(GIT_DIR)/gitweb' directory.

Signed-off-by: Pavan Kumar Sunkara <pavan.sss1991@gmail.com>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Petr Baudis <pasky@ucw.cz>
Acked-by: Jakub Narebski <jnareb@gmail.com>
Acked-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>