diff: preparse --diff-filter string argument

Instead of running strchr() on the list of status characters over
and over again, parse the --diff-filter option into bitfields and
use the bits to see if the change to the filepair matches the status
requested.

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

diff.h

@@ -103,12 +103,15 @@ enum diff_words_type {
 };
 
 struct diff_options {
-	const char *filter;
 	const char *orderfile;
 	const char *pickaxe;
 	const char *single_follow;
 	const char *a_prefix, *b_prefix;
 	unsigned flags;
+
+	/* diff-filter bits */
+	unsigned int filter;
+
 	int use_color;
 	int context;
 	int interhunkcontext;