docs: def find_revision_id(self, revision=None):
          """Find the global revision id of the given revision."""
          # Make sure the local repository exists.
          self.create()
          # Try to find the revision id of the specified revision.
          revision = revision or self.default_revision
          output = self.context.capture(
              'bzr', 'version-info', '--revision=%s' % revision,
              '--custom', '--template={revision_id}',
          )
          # Validate the `bzr version-info' output.
          if not output:
              msg = "Failed to find global revision id! ('bzr version-info' gave unexpected output)"
              raise ValueError(msg)
          return output