gitweb: Support for 'forks'

gitweb: Support for 'forks'

On repo.or.cz, I want to support project 'forks', which are meant
for repositories which are spinoffs of a given project and share
its objects database through the alternates mechanism. But another
(and perhaps even greater) incentive for that is that those 'forked
projects' do not clutter the main project index but are completely
grouped inside of the project view.

A forked project is just like a normal project, but given project
$projectroot/$projname.git, the forked project resides in directory
$projectroot/$projname/$forkname.git. This is a somewhat arbitrary
naming rule, but I think that for now it's fine; if someone will need
something wildly different, let them submit a patch. The 'forked'
mode is by default off and can be turned on in runtime gitweb
configuration just like other features.

A project having forks is marked by a '+' (pointing to the list of
forks) in the project list (this could become some cutesy AJAXy
DHTML in the future), there is a forks section in the project
summary similar to the heads and tags sections, and of course
a forks view which looks the same as the root project list.

Forks can be recursive.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>