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
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors