Mark individual changes as approved/read in a pull request

As a developer
I would like to approve portions of a pull request

so that I
can ignore changes I've already seen/approved on future updates to the pull request.

Currently: Sometimes I'll be reviewing a large pull request that spans many files. I will add comments. The author will then reply to the comments make changes and push them up to BitBucket. Then I'll review and make more comments. Rinse/repeat. Each time I re-review, I have to go through all the changes I've already seen in the off-chance that I've missed something.

Proposal: I would love to mark individual changes and/or entire files as having been approved/read. If further updates to the pull request don't affect those changes/files, then rather than seeing the red/green difference, perhaps they show up as some other color (grey?). This would allow me to easily focus on the new changes to the pull request since I last viewed it.

I posted this wish to my team HipChat and it was met with a resounding yes! I have a feeling this would be a very popular feature.