Refine the update heuristic to improve responsiveness a bit.

Refine the update heuristic to improve responsiveness a bit.

The previous commit improved performance a lot but also meant that
we waited longer to see something drawn.  This refines the heuristics
for when to call update so that (1) when we have finished processing
a bufferfull of information from git-rev-list, we call update if
enough time has elapsed, regardless of how many commits we've drawn,
and (2) the number of commits drawn between updates scales with the
total number of commits drawn: 1 for 1-99 commits, 10 for 100-9999
commits, or 100 for >= 10000 commits.