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
$ checkflake8() {
> if ! (which flake8 > /dev/null); then
> echo skipped: missing tool: flake8;
> exit 80;
> fi;
> };
$ checkflake8
Copied from Mercurial core (60ee2593a270)
$ cd "`dirname "$TESTDIR"`"
run flake8 if it exists; if it doesn't, then just skip
$ type flake8 >/dev/null 2>/dev/null && hg files -0 'glob:**.py' | xargs -0 flake8