gitweb: Even more support for PATH_INFO based URLs

gitweb: Even more support for PATH_INFO based URLs

Now the following types of path based URLs are supported:

* project              overview (summary) page of project
* project/branch       shortlog of branch
* project/branch:file  file in branch, blob_plain view
* project/branch:dir/  directory listing of dir in branch, tree view

The following shortcuts works (see explanation below):

* project/branch:      directory listing of branch, main tree view
* project/:file        file in HEAD (raw)
* project/:dir/        directory listing of dir in HEAD
* project/:            directory listing of project's HEAD

We use ':' as separator between branch (ref) name and file name
(pathname) because valid branch (ref) name cannot have ':' inside.
This limit applies to branch name only. This allow for hierarchical
branches e.g. topic branch 'topic/subtopic', separate remotes
tracking branches e.g. 'refs/remotes/origin/HEAD', and discriminate
between head (branch) and tag with the same name.

Empty branch should be interpreted as HEAD.

If pathname (the part after ':') ends with '/', we assume that pathname
is name of directory, and we want to show contents of said directory
using "tree" view. If pathname is empty, it is equivalent to '/' (top
directory).

If pathname (the part after ':') does not end with '/', we assume that
pathname is name of file, and we show contents of said file using
"blob_plain" view.

Pathname is stripped of leading '/', so we can use ':/' to separate
branch from pathname. The rationale behind support for PATH_INFO based
URLs was to support project web pages for small projects: just create
an html branch and then use an URL like
  http://nowhere.com/gitweb.cgi/project.git/html:/index.html
The ':/' syntax allow for working links between .html files served
in such way, e.g. <a href="main.html"> link inside "index.html"
would get
  http://nowhere.com/gitweb.cgi/project.git/html:/main.html.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>