Fix for x/-2 appearing as \frac{x}{2} (no negative sign).

Made sure the right hint gets added in this particular case.
So glad that switching to MathQuill will mean we can get rid of most (all?)
rendering related hints, because I never liked that system very much.

Also, didn't forget to build this time. :)

Test Plan:
New test cases, no more failures!

Auditors: jack