Replaces "subpath" with "branch"

In favor of a more widely used varaible, the subpath variable has
been replaced with branch. In preparation for branch support in
overlay.xml files.

layman.8.txt: An example of an overlay.xml with a branch in it has
been added for users along with pointers to using the variable.
{cvs, tar}.py: Fixes assignment for self.branch

Fixes commit: fix undefined 'subpath', update tests
SHA:0fde69e7ef993c3e5cca925cef35c8c895cfd834
Author: dol-sen <brian.dolbec@gmail.com>
Date: Sat 23 Jul 2011 02:44:26 AM EDT

Fixes commit: removal of xml from the vcs class parameters.
SHA: ba5f0cd5c948bb7a65ab21008c940caf0fbe38e7
Author: Brian Dolbec <brian.dolbec@gmail.com>
Date: Thu 24 Feb 2011 01:49:58 AM EST
darcs.py: Adds module branch support
git.py: Adds module branch support
mercurial.py: Adds module branch support