Column labels with --full

[Dima Kogan]

Hi. In writing tests for the vnlog support I stumbled on what looks like
a bug. Or at the very least an odd behavior. There's even a test for it,
but the "reference" output we compare against looks wrong.

I'm looking at the hdr4 and hdr5 tests:

  
https://git.savannah.gnu.org/cgit/datamash.git/tree/tests/datamash-tests.pl?h=v1.7#n541

Both process this input data:

  A 3 W
  A 5 W
  A 7 W
  A 11 X
  A 13 X
  B 17 Y
  B 19 Z
  C 23 Z

Test hdr4 runs essentially "datamash --group 1 --header-out count 2"
with this expected result:

  GroupBy(field-1) count(field-2)
  A 5
  B 2
  C 1

Which is fine. The next test (hdr5) does the same thing, but adds
"--full". I would expect the same output as in hdr4, but with the extra
columns we just threw out being re-added. This isn't entirely what
happens:

  field-1 field-2 field-3 count(field-2)
  A 3 W 5
  B 17 Y 2
  C 23 Z 1

Note that the field label for our grouping key changed from
"GroupBy(field-1)" to "field-1". I don't think this is right. Opinions?