Documentation/git-for-each-ref: don't list date formats

Documentation/git-for-each-ref: don't list date formats

We are about to add a new set of supported date formats and do not want
to have to maintain the same list in several different bits of
documentation.  Refer to git-rev-list(1) which contains the full list of
supported formats.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>