CG: explanation of branches for the impatient (issue 5484043)

[graham]

Reviewers: ,

Message:
CG now has N+1 different introductory explanations of git, where N is at
least 3.  But given the lack of familiarity with branches that I see in
our development community, and the new staging stuff, I think this one
is good.

Once it's been accepted (in whatever form), I'll clean up the rest of
this chapter.

Description:
CG: explanation of branches for the impatient

Please review this at http://codereview.appspot.com/5484043/

Affected files:
  M Documentation/contributor/source-code.itexi