gitweb: Make the `blame' interface in gitweb optional.

Since `git-annotate' is an expensive operation to run it may be
desirable to deactivate this functionality. This patch introduces
the `gitweb.blame' option to git-repo-config and disables the blame
support by default.

Signed-off-by: Florian Forster <octo@verplant.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>