Proposal: Add review tags to patch review workflow.

[Simon Marchi]

>> It also wasn't clear to me if the intention was for the commits to
>> be amended with the annotations?  (I don't think it was explicitly
>> stated in the original mail, and I'm not sure if it was an implicit
>> assumption?) 
> No, I didn't intend on amending previous commits. The main problem this change intends to solve is fixing ambiguity, and the pushed patches don't have that issue anymore. Thanking a reviewer is just one more positive side (IMHO) going forward.

Err, just to be clear, we won't amend existing commits in master
obviously (can't rewrite history) but future commits in master would
contain those tags.  This means that if you give me a Reviewed-By, I
amend my local commit patch to include that trailer before pushing.

Simon