revlog: drop cache after use to save memory footprint
If we reconstruct back to back large versions, we need to drop the cache first to avoid doubling memory usage.
Ancestors of 54 2 0 Ancestors of 6 and 53 4 2 1 0 Ancestors of 5 and 44 2 0 Descendants of 57 8 Descendants of 5 and 36 7 8 Descendants of 5 and 45 7 8