t7406: fix misleading "rev-parse --max-count=1 HEAD"
The test happened to use "rev-parse --max-count=1 HEAD" consistently
to prepare the expected output and the actual output, so the
comparison between them gave us a correct success/failure because
both output had irrelevant "--max-count=1" in it.
But that is not an excuse to keep it broken. Replace it a more
meaningful construct "rev-parse --verify HEAD".
Noticed by Daniel GraƱa while working on his submodule tests.
Signed-off-by: Junio C Hamano <gitster@pobox.com>