revlog: limit base to rev size ratio to 500 instead of 50
While a value of 50 provided large speedup in some case (400s → 7s) it also
creates a slow down for a whole class of revision we are seeing in a private
repository (0.1s → 3s). A value of 500 makes them disappear improving the
total runtime (the slower revision still improve significantly (400s → 21s)).
We need to run a wider array of tests on various repositories to see the
effect on speed and size of different values for this acceptable constant.
However, in the meantime, it seems safer to move back to a less impactful