documentation: add tutorial for object walking

documentation: add tutorial for object walking

Existing documentation on object walks seems to be primarily intended
as a reference for those already familiar with the procedure. This
tutorial attempts to give an entry-level guide to a couple of bare-bones
object walks so that new Git contributors can learn the concepts
without having to wade through options parsing or special casing.

The target audience is a Git contributor who is just getting started
with the concept of object walking. The goal is to prepare this
contributor to be able to understand and modify existing commands which
perform revision walks more easily, although it will also prepare
contributors to create new commands which perform walks.

The tutorial covers a basic overview of the structs involved during
object walk, setting up a basic commit walk, setting up a basic
all-object walk, and adding some configuration changes to both walk
types. It intentionally does not cover how to create new commands or
search for options from the command line or gitconfigs.

There is an associated patchset at
https://github.com/nasamuffin/git/tree/revwalk that contains a reference
implementation of the code generated by this tutorial.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>