Fix logic in doc impact

Previous patch was supposed to update notify_impact
so that doc impact flags only triggered bugs on change
merge. This worked, however the branch logic was flawed,
meaning that non-merging changes now trigger emails.

This patch fixes for the desired behaviour

Change-Id: I3b028319c026bce84d4641e417aed9e8e0f52063
Merge "Fix logic in doc impact"