stablerange: use the new cache object in the 'mergepoint' version
Every improvement to the cache should reflect on the stable range from there.
Since we no longer use the basic sort function, we can simplify the inheritance
by dropping one layer.
#require test-repo pyflakes
Copied from Mercurial core (60ee2593a270)
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null