docs: Add a document on how to fix a messy diffstat

A branch with merges in will sometimes create a diffstat containing a lot
of unrelated work at "git request-pull" time.  Create a document based on
Linus's advice (found in the links below) and add it to the maintainer
manual in the hope of saving some wear on Linus's keyboard going forward.

Link: https://lore.kernel.org/lkml/CAHk-=wg3wXH2JNxkQi+eLZkpuxqV+wPiHhw_Jf7ViH33Sw7PHA@mail.gmail.com/
Link: https://lore.kernel.org/lkml/CAHk-=wgXbSa8yq8Dht8at+gxb_idnJ7X5qWZQWRBN4_CUPr=eQ@mail.gmail.com/


Acked-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>