There have been various proposals over the years for how to integrate issues and reviews in the distributed git tree itself (http://dist-bugs.branchable.com/software/), but I don't think any of them have really gone anywhere, certainly not in terms of support by the hosted git vendors.

Having looked briefly into it now, git-dit does look promising in its approach. I'd be interested to hear from someone who had actually used it and bumped up against the limitations: https://github.com/neithernut/git-dit/blob/master/doc/datamo...