gitweb: start to generate PATH_INFO URLs.

gitweb: start to generate PATH_INFO URLs.

Instead of providing the project as a ?p= parameter it is simply appended to
the base URI.  All other parameters are appended to that, except for ?a=summary
which is the default and can be omitted.

The this can be enabled with the "pathinfo" feature in gitweb_config.perl.

[jc: let's introduce new features disabled by default not to
 upset too many existing installations.]

Signed-off-by: Martin Waitz <tali@admingilde.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>