stablerange: compute subranges from parent when possible
Unless we are a merge, we can infer the standard subranges of a range by reusing
the subranger of its parent. We update the implementation to do so.
We still needs the "old way" for merge. We move that code in a dedicated
function for the sake of simplicity.
#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