checkheahds: switch algorithm to use pushed markers instead
We now checks if markers involved in the push are relevant to nodes in the
branch we try to replace. The approach is simpler and more robust.
A test showing the limitation of the previous approach is added.
#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