docs: def patches_base_ref(default=exception.CantGuess):
      """Return a git reference to patches branch base.
  
      Returns first part of .spec's patches_base is found,
      otherwise return Version(+%{milestone}).
      """
      ref = None
      try:
          spec = specfile.Spec()
          ref, _ = spec.get_patches_base(expand_macros=True)
          if ref:
              ref, _ = tag2version(ref)
          else:
              ref = spec.get_tag('Version', expand_macros=True)
              milestone = spec.get_milestone()
              if milestone:
                  ref += milestone
          if not ref:
              raise exception.CantGuess(msg="got empty .spec Version")
      except Exception as ex:
          if default is exception.CantGuess:
              raise exception.CantGuess(
                  what="current package version",
                  why=str(ex))
          else:
              return default
      tag_style = version_tag_style(ref)
      return version2tag(ref, tag_style=tag_style)