revset: predicate to avoid lookup errors A query like head() and (descendants("bad") and not descendants("fix")) (testing if repo heads are affected by a bug) will abort with a RepoLookupError if either badrev or fixrev aren't found inside the repository, which is not very informative. The new predicate returns an empty set for lookup errors, so head() and (descendants(present("bad")) and not descendants(present("fix"))) will behave as wanted even if those revisions are not found.