When a user is scrolling through the changeset (post commit review) to 

make a new review request on the web UI, clicking on the row of a 

commit makes a new review request, which can be done accidentally very 

easily. 
I added a confirmation dialog when creating post-commit review requests 

on web UI so the user can confirm that they want to create a new review 

request from that commit. Clicking 'cancel' closes the dialog and takes 

no further action, clicking 'create' creates a new review request.

Ran JS tests (all of which pass).
Tested behaviour in Chrome, Safari, and Firefox with the following 

scenarios:

Click on commit, click cancel on confirmation dialog
Click on commit with no diff with master, click confirm, see error 

message
Click on commit with diff with master, click confirm, see review 

request created