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
% lazy ancestor set for [], stoprev = 0, inclusive = False
membership: []
iteration: []
% lazy ancestor set for [11, 13], stoprev = 0, inclusive = False
membership: [7, 8, 3, 4, 1, 0]
iteration: [3, 7, 8, 1, 4, 0, 2]
% lazy ancestor set for [1, 3], stoprev = 0, inclusive = False
membership: [1, 0]
iteration: [0, 1]
% lazy ancestor set for [11, 13], stoprev = 0, inclusive = True
membership: [11, 13, 7, 8, 3, 4, 1, 0]
iteration: [11, 13, 3, 7, 8, 1, 4, 0, 2]
% lazy ancestor set for [11, 13], stoprev = 6, inclusive = False
membership: [7, 8]
iteration: [7, 8]
% lazy ancestor set for [11, 13], stoprev = 6, inclusive = True
membership: [11, 13, 7, 8]
iteration: [11, 13, 7, 8]