mcenc.c: Properly clear MVs hit cache

This avoids spurious hits when hit_bit is reset to 0.
Unfortunately this slightly increases rate on video-1-short:
           RATE (%)  DSNR (dB)
    PSNR 0.03606  -0.00193
 PSNRHVS 0.03931  -0.00311
    SSIM 0.01739  -0.00084
FASTSSIM 0.02251  -0.00138