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>
diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index 6e26832..7788d4f 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -65,6 +65,9 @@
 	can be set with "--dirstat=limit". Changes in a child directory is not
 	counted for the parent directory, unless "--cumulative" is used.
 
+--dirstat-by-file[=limit]::
+	Same as --dirstat, but counts changed files instead of lines.
+
 --summary::
 	Output a condensed summary of extended header information
 	such as creations, renames and mode changes.