This matches how the diff Porcelain works. It makes the plumbing commands
respect diff's configuration options, such as indentHeuristic, because
init_revisions() calls diff_setup() which fills in the diff_options struct.
Signed-off-by: Marc Branchaud <marcnarc@xiplink.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>