git-codereview: add a hint about mailing the whole chain of changes
git-codereview: add a hint about mailing the whole chain of changes

"mail HEAD" will mail every commit between HEAD and branching point.

Fixes golang/go#16034

Change-Id: If95aece584f2a324f866c64770f7361ddb3e7636
Reviewed-on: https://go-review.googlesource.com/c/review/+/148137
Reviewed-by: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>

golang/review