If the user provides an empty format with "--format=", we
end up putting in extra whitespace that the user cannot
prevent. This comes from two places:
1. If the format is missing a terminating newline, we add
one automatically. This makes sense for --format=%h, but
not for a truly empty format.
2. We add an extra newline between the pretty-printed
format and a diff or diffstat. If the format is empty,
there's no point in doing so if there's nothing to
separate.
With this patch, one can get a diff with no other cruft out
of "diff-tree --format= $commit".
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>