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.
#!/bin/sh
hg init
echo a > a
hg add
hg ci -m1
# set mtime of a into the future
touch -t 202101011200 a
# status must not set a's entry to unset (issue1790)
hg status
hg debugstate