diff.h: make pickaxe_opts an unsigned bit field

This variable is used as a bit field[1], and as we are about to add more
fields, indicate its usage as a bit field by making it unsigned.

[1] containing the bits

    #define DIFF_PICKAXE_ALL	1
    #define DIFF_PICKAXE_REGEX	2
    #define DIFF_PICKAXE_KIND_S	4
    #define DIFF_PICKAXE_KIND_G	8

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.h

@@ -146,7 +146,7 @@ struct diff_options {
 	int skip_stat_unmatch;
 	int line_termination;
 	int output_format;
-	int pickaxe_opts;
+	unsigned pickaxe_opts;
 	int rename_score;
 	int rename_limit;
 	int needed_rename_limit;