system_config: print brew branch

[gromgit]

When debugging others' problems (or even one's own), it helps to know which branch brew itself is on, to avoid unwarranted assumptions that lead to a wild goose chase. (Been there, done that, facepalmed more than once.)

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

[MikeMcQuaid]

Thanks @gromgit! Good idea, just think we'll tweak slightly before merge.

[MikeMcQuaid]

Thanks @gromgit!