That sort of "inline" issue tracking is a thing. I think Bugs Everywhere[1] is one of the more mature systems based on the idea. There are several others too[2], most of them unmaintained. There are also wiki-style systems based on the same idea.

[1] http://www.bugseverywhere.org/ [2] http://www.cs.unb.ca/~bremner/blog/posts/git-issue-trackers/