[$] Handling messy pull-request diffstats

Subsystem maintainers routinely use gitrequest-pull as part of the process of sending work upstream. Normally, the result includes a list ofcommits included in the request and a nicediffstat that shows which files will be touched and how much of each willbe changed; examplesabound on the kernel mailing lists. Occasionally, though, a repository with a relativelycomplicated development history will yield a massive diffstat containing agreat deal of unrelated work. The result looks ugly and obscures what thepull request is actually doing. This document describes what is happeningand how to fix things up; it is derived from The Wisdom of Linus Torvalds,which has been posted numerous times over the years (example 1,example 2).