diffcore_filespec: add is_binary

diffcore-break and diffcore-rename would want to behave slightly
differently depending on the binary-ness of the data, so add one
bit to the filespec, as the structure is now passed down to
diffcore_count_changes() function.

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

diffcore.h

@@ -37,6 +37,7 @@ struct diff_filespec {
 #define DIFF_FILE_VALID(spec) (((spec)->mode) != 0)
 	unsigned should_free : 1; /* data should be free()'ed */
 	unsigned should_munmap : 1; /* data should be munmap()'ed */
+	unsigned is_binary : 1; /* data should be considered "binary" */
 };
 
 extern struct diff_filespec *alloc_filespec(const char *);