[Zambyte]

I'm not really sure what you mean by this. Git treats commits as atomic (unchangeable), and thus they are atoms (indivisible). This is not really related to patches, unless you're referring to partially applying patch series. Can you elaborate on what you mean?

[hinkley]

This guy explains it better than I could

https://news.ycombinator.com/item?id=44355218

[cma]

Does the commit message itself affect the hash or is it separate and outside of the main system, like tags?

[Zambyte]

The commit message affects the hash.