Use newly-introduced finely-grained control to teach the diff-family to
honor the new environment GIT_PRINT_SHA1_ELLIPSIS and remove the
ellipses when it is not set.
Mentored-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ann T Ropea <bedhanger@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>