diff --dirstat-by-file: count changed files, not lines

diff --dirstat-by-file: count changed files, not lines

This new option --dirstat-by-file is the same as --dirstat, but it
counts "impacted files" instead of "impacted lines" (lines that are
added or removed).

Signed-off-by: Heikki Orsila <heikki.orsila@iki.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>