add rule for better reviewing, don't amend new changes after diff accepted, new a diff for that.

Summary: amend changes to a accepted diff is dangerous, which will make the codebase out of control.

Test Plan: NA

Reviewers: #bitcoin_abc, O1 Bitcoin ABC, schancel

Reviewed By: #bitcoin_abc, O1 Bitcoin ABC, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1270