stablerange: extract the core API into a 'stablerangecore' class
To clarify what the code is doing without all the caching and algorithmic
improvement, we extract a core class that highlight the basic API expected from
the class.
#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