discovery: introduce "stable slicing" methods
We introduce new code that leverage the stable sorting to slices a graph in a
way "stable" accross repository. This should allow us to use theses slices for
obsolescence markers discovery.
#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")' 2>/dev/null \
> | xargs pyflakes 2>/dev/null