diff family: add --check option

Actually, it is a diff option now, so you can say

	git diff --check

to ask if what you are about to commit is a good patch.

[jc: this also would work for fmt-patch, but the point is that
 the check is done before making a commit.  format-patch is run
 from an already created commit, and that is too late to catch
 whitespace damaged change.]

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.h

@@ -153,6 +153,7 @@ extern int diff_queue_is_empty(void);
 #define DIFF_FORMAT_NAME	4
 #define DIFF_FORMAT_NAME_STATUS	5
 #define DIFF_FORMAT_DIFFSTAT	6
+#define DIFF_FORMAT_CHECKDIFF	7
 
 extern void diff_flush(struct diff_options*);