Fix pre-enable engagement on GM (#348)

In 2017 Volts, PCM fault occurs for a few seconds if ACC gas
is commanded while user presses gas pedal. PID winds up,
and when PCM fault clears, car gets a "max gas" jolt.

In 2018 Volts, PCM fault doesn't time out, which means pre-enable
doesn't work at all, and car would slowly decelerate, while openpilot
thinks it's engaged.

selfdrive/car/gm/interface.py

@@ -325,7 +325,11 @@ class CarInterface(object):
       "chimeRepeated": (CM.LOW_CHIME, -1),
       "chimeContinuous": (CM.LOW_CHIME, -1)}[str(c.hudControl.audibleAlert)]
 
-    self.CC.update(self.sendcan, c.enabled, self.CS, self.frame, \
+    # For Openpilot, "enabled" includes pre-enable.
+    # In GM, PCM faults out if ACC command overlaps user gas.
+    enabled = c.enabled and not self.CS.user_gas_pressed
+
+    self.CC.update(self.sendcan, enabled, self.CS, self.frame, \
       c.actuators,
       hud_v_cruise, c.hudControl.lanesVisible, \
       c.hudControl.leadVisible, \