Log Message:

pkg_cmp_summary compares files that contain summary information
about packages (pkg_summary.txt format).
These files may be generated by pkg_info -X, pkg_src_summary
or anything else.
Actually PKGNAME fields are compared only.
All PKGNAMEs are checked, i.e. versions are compared and
presense is checked.

Format of the results:
  <R> <PKGBASE> <VER1> <VER2>
where R is
  -     present in the first file but absent in the second one (disappears)
  +     absent in the first file but present in the second one (appears/new)
  <NUM> more than one package with the same PKGBASE is in the first file
  =     versions are the same
  <     second file contains more recent version (upgrade is possible)
  >     first file contains earlier version (downgrade is possible)
  ?     versions are incomparable (e.g. 1.2.3 vs. 20080101)

PKGBASE is PKGNAME with version stripped,
VER1 and VER2 are package versions. VER2 is present only if
   R is `<' or `>'