The "Force Commit" button that comes down in a sheet if the pre-commit hook fails does nothing (as of 0.64). In the brotherbard fork, this feature did work, so I'd consider this some kind of regression.
Seems reasonable; I don't think I've tested with any pre-commit hooks, myself.
I can confirm this bug. I've just upgraded from gitx(L) where I used it regularly, but it's not working for me now.