rebase: inline simple function for finding obsolete subset of commits
_filterobsoleterevs() is just one line long. It was introduced in
2d294dada4f8 (rebase: small refactoring to allow better extensibility
from extensions, 2016-01-14), for use by the "inhibit" extension. That
extension was removed from the evolve repo in 87e87881059d (compat:
drop the inhibit hacky extension, 2017-10-24).
Differential Revision: https://phab.mercurial-scm.org/D10198