definitive git instructions

[Graham Percival]

We have a bunch of different "getting started with git"
instructions floating around.  The main "development" page talks
about git clone; GDP suggested "git init; git remote add...; git
checkout"; the README in web/ (and now web-gop/) branches talks
about "git init-db; ... set up .git/remotes/...; git fetch; git
checkout -b"


I want to clear this up and get the first piece of new content for
the new GOP website.  With a modern version of git (whatever that
means), what's the recommended way to get:

1.  The master branch of the source tree.

2.  All branches of lilypond.

Is there much of a difference (in download size) between #1 and
#2?  If somebody wants to fix bugs and update the website, is it
better to grab all the branches (is that clone?), or keep two
separate source directories (~/src/lilypond-main/ and
~/src/lilypond-web/ )?

Cheers,
- Graham