Ability to comment on branches

I would like to be able to add comments per line or file or commit within the origin of a branch.
On many occasions, people who write code would love some collaborative feedback on a large piece of code before it is ready to be merged into a master branch.
In most cases, one can speak directly with the colleague who pushed the changes or the colleague can make a pull request notifying everyone this is not meant for merger yet.
But where different time zones and work from home situations exist then problems tend to occur.
With the current solution of direct contact about pushed code, I cannot speak with the colleague until later, by which time I might forget some of the detail, or not have the time later. But as I am notified of push on the branch, I can comment immediately on questionable lines, files, and commits quickly. I also won't forget what I wished to have said in the period between noting it and the availability of the colleague.
If the solution of a pull request but not merged is used then the possibility is that everyone believes this intermediate code looks good, maybe with minor comments, then somebody approves it and merges because they were not aware of the exceptional circumstance.

Apologies if there is already a good solution for this type of scenario, but it does seem strange that I cannot comment on all origin code.