stack: use stack._revs instead of stack.revs[1:] in external children revset
The revset in question excludes all revs that are part of the stack. Using
stack.revs[1:] works (rev #0 is stack base, which is not a part of the stack),
but using stack._revs is technically more correct, because this variable is
specifically designed to hold only revisions that are part of the stack.
#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 -I 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null