There is one git client that suits all your requirements - it's the git client integrated in Eclipse IDE - I believe it's called EGit. It's the ONLY GUI client that I've found that has ability to edit hunks (you can actually edit whatever you want, even without adding any hunk/line). It only has one downside, which is... Eclipse (; OK, two downsides - ...
Take a look at Gitea (basically a clone of GitHub).
It should have all the features you are searching for:
Integration with Active Directory through LDAP
Pull requests with comments
I'm not sure about the granularity of users & roles, but you can try it yourself on the demo hosted here: