git-instaweb: Put httpd logs in a "$httpd_only" subdirectory

git-instaweb: Put httpd logs in a "$httpd_only" subdirectory

Resolve full httpd and create "$httpd_only" subdirectory before
writing httpd.conf so that error.log and access.log go into it.

While at it, change apache2 configuration to use logs in a
similiar fashion.

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: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>