diff: add --default-prefix option

You can change the output of prefixes with diff.noprefix and
diff.mnemonicprefix, but there's no easy way to override them from the
command-line. We do have "--no-prefix", but there's no way to get back
to the default prefix. So let's add an option to do that.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/diff.c b/diff.c
index 750d1b1..b322e31 100644
--- a/diff.c
+++ b/diff.c
@@ -5275,6 +5275,17 @@ static int diff_opt_no_prefix(const struct option *opt,
 	return 0;
 }
 
+static int diff_opt_default_prefix(const struct option *opt,
+				   const char *optarg, int unset)
+{
+	struct diff_options *options = opt->value;
+
+	BUG_ON_OPT_NEG(unset);
+	BUG_ON_OPT_ARG(optarg);
+	diff_set_default_prefix(options);
+	return 0;
+}
+
 static enum parse_opt_result diff_opt_output(struct parse_opt_ctx_t *ctx,
 					     const struct option *opt,
 					     const char *arg, int unset)
@@ -5564,6 +5575,9 @@ struct option *add_diff_options(const struct option *opts,
 		OPT_CALLBACK_F(0, "no-prefix", options, NULL,
 			       N_("do not show any source or destination prefix"),
 			       PARSE_OPT_NONEG | PARSE_OPT_NOARG, diff_opt_no_prefix),
+		OPT_CALLBACK_F(0, "default-prefix", options, NULL,
+			       N_("use default prefixes a/ and b/"),
+			       PARSE_OPT_NONEG | PARSE_OPT_NOARG, diff_opt_default_prefix),
 		OPT_INTEGER_F(0, "inter-hunk-context", &options->interhunkcontext,
 			      N_("show context between diff hunks up to the specified number of lines"),
 			      PARSE_OPT_NONEG),