Allow reviewer to set 'accepted' status for parts of a merge request

Description including problem, use cases, benefits, and/or goals

When reviewing large merge requests I find myself loosing a lot of time sifting through many lines of code that I've seen before and basically approved of.

I'm reviewing a huge merge request. I went through some source files that I was OK with, made comments on some other files. Now I get interrupted for a few hours, or maybe a night of sleep. When I return to the reviewing task, I find that the submitter made some changes, perhaps already fixing some of my comments. Maybe also changing some other stuff. Now it becomes a bit problematic for me to know if all the code I already went through is still OK. Also, since the reviewing doesn't happen linearly from the first change to the last, but for example follows the execution flow, I might run into some sections where I'm not sure if I've already reviewed it two days ago.

Proposal

Have the possibility to 'check off' files, code blocks and maybe even individual lines of code, that I already went through so that I immediately know what parts still need to be reviewed - and also what parts that I already reviewed have changed so that I need to review again, and wonder why they were changed.

~"feature proposal" ~"Accepting Merge Requests"