user-manual: move howto/using-topic-branches into manual

Move howto/using-topic-branches into the user manual as an example for
the "sharing development" chapter.  While we're at it, remove some
discussion that's covered in earlier chapters, modernize somewhat (use
separate-heads setup, remotes, replace "whatchanged" by "log", etc.),
and replace syntax we'd need to explain by syntax we've already covered
(e.g. old..new instead of new ^old).

The result may not really describe what Tony Luck does any more.... Hope
that's not annoying.

Cc: Tony Luck <tony.luck@intel.com>
Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>