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