convert: fix the RevisionSpec import in the bzr module This import was failing (because its invalid) which was resulting in the following tests failing with chg running: - test-convert-bzr.t - test-convert-bzr-directories.t - test-convert-bzr-ghosts.t - test-convert-bzr-treeroot.t - test-convert-bzr-114.t - test-convert-bzr-merges.t This commit fixes the import which in turn fixes the tests. Test Plan: Ran the aforementioned tests with and without '--chg' option. Differential Revision: https://phab.mercurial-scm.org/D936