revlog: add a C implementation of `headrevsdiff` Python implementation of `headrevsdiff` can be very slow in the worst case compared with the `heads` computation it replaces, since the latter is done in C. Even the average case of this Python implementation is still noticeable in the profiles. This patch makes the computation much much faster by doing it in C.

unbundle: faster computation of changed heads To compute the set of changed heads it's sufficient to look at the recent commits, instead of looking at all heads currently in existence.