Disallow using error-prone merge strategies

DO NOT MERGE / RESTRICT AUTOMERGE and its cousin DO NOT MERGE
ANYWHERE are almost always misused. Make it an error to upload
commits with those merge strategies.

Bug: 156896646
Test: repo upload -n (with DNM and RM commits)
Change-Id: Iae3540fa35dbd7b87a14b53a20be0cd23a206f3a