compat: fix instabilities compat
We added an instabilities method for old version of Mercurial. But we returned
an old value for orphan changesets, the one before the big evolve renaming.
Fix the return value to match the new return values.
#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