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>

Documentation/diff-options.txt

@@ -852,6 +852,11 @@ endif::git-format-patch[]
 --no-prefix::
 	Do not show any source or destination prefix.
 
+--default-prefix::
+	Use the default source and destination prefixes ("a/" and "b/").
+	This is usually the default already, but may be used to override
+	config such as `diff.noprefix`.
+
 --line-prefix=<prefix>::
 	Prepend an additional prefix to every line of output.
 

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),

t/t4013-diff-various.sh

@@ -643,9 +643,19 @@ test_expect_success 'diff respects diff.noprefix' '
 	check_prefix actual file0 file0
 '
 
+test_expect_success 'diff --default-prefix overrides diff.noprefix' '
+	git -c diff.noprefix diff --default-prefix >actual &&
+	check_prefix actual a/file0 b/file0
+'
+
 test_expect_success 'diff respects diff.mnemonicprefix' '
 	git -c diff.mnemonicprefix diff >actual &&
 	check_prefix actual i/file0 w/file0
 '
 
+test_expect_success 'diff --default-prefix overrides diff.mnemonicprefix' '
+	git -c diff.mnemonicprefix diff --default-prefix >actual &&
+	check_prefix actual a/file0 b/file0
+'
+
 test_done