topic: reorder revset for faster evaluation
When running the revset on unfiltered repository, the mutable set can get huge.
So removing the obsolete changeset first provide a large speedup. This resolve
the performance regression we were seeing since the 6.7.0 release.
#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