diff --dirstat: saner handling of binary and unmerged files

We do not account binary nor unmerged files when --shortstat is
asked for (or the summary stat at the end of --stat).

The new option --dirstat should work the same way as it is about
summarizing the changes of multiple files by adding them up.

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