Unified Octave Forge repository -- includes all Octave Forge packages

[Carnë Draug]

Hi everyone

I have written a Makefile that can be used to keep a clone of all Octave
Forge repositories around and up to date.  It started as a series of hg
subrepos but turns out that is not only overkill, it's the whole wrong tool
for the job (thank you Jordi, jwe, and mtmx).

Clone it:

    hg clone http://hg.code.sf.net/p/octave/octave-forge

How to use it:

    cd octave-forge

    ## clone any package by name
    make video

    ## update any package by name (clone it if necessary)
    make update-signal

    ## update ALL repos (and clone them ALL if necessary)
    ## WARNING: it may take a long time
    make update

  ## update all in parallel
  make update -j8

It is very simple on purpose and assumes cleans repositories for the update.
I want to avoid project specific code for the update.

Because it would very easy to accidentally add new files to this repo (if
you plan to use it as the basis of your work), I have made it read-only for
everyone.

Enjoy.

Carnë