gitweb: more support for PATH_INFO based URLs

gitweb: more support for PATH_INFO based URLs

Now three types of path based URLs are supported:
	gitweb.cgi/project.git
	gitweb.cgi/project.git/branch
	gitweb.cgi/project.git/branch/filename

The first one (show project summary) was already supported for a long time
now.  The other two are new: they show the shortlog of a branch or
the plain file contents of some file contained in the repository.

This is especially useful to support project web pages for small
projects: just create an html branch and then use an URL like
gitweb.cgi/project.git/html/index.html.

Signed-off-by: Martin Waitz <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>