evolve: move _orderrevs() function evolvecmd.py
_orderrevs() is a function which is used to order the revs in which the
instability needs to be solved if we have multiple revisions to resolve.
#require test-repo pyflakesCopiedfromMercurialcore(60ee2593a270)$cd"`dirname "$TESTDIR"`"runpyflakesonalltrackedfilesendingin.pyorwithoutafileending(skippingbinaryfilerandom-seed)$hglocate'set:(**.py or grep("^#!.*python")) - removed()'2>/dev/null \>|xargspyflakes2>/dev/null