Allow users to detail changes between successive patch submission rounds

[rtyley]

On Fri, Mar 11, 2016 at 4:31 AM, Eric Sunshine <sunshine sunshineco.com> wrote:
As a convenience to reviewers, please use this area below the "---"
line to provide links and explain what changed since the previous
round rather than doing so in a separate email.

http://article.gmane.org/gmane.comp.version-control.git/288674

Noted during the submission of git/git#205