t7406: fix misleading "rev-parse --max-count=1 HEAD"

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 Graña while working on his submodule tests.

Signed-off-by: Junio C Hamano <gitster@pobox.com>