Documentation: update "howto maintain git"

The flow described in the document is still correct, but over time I
have automated various parts of the workflow with tools and their
use was not explained at all.

Update it and outline the use of two key scripts from the 'todo'
branch, "Reintegrate" and "cook".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: gitster