diff: add diff.srcPrefix and diff.dstPrefix configuration variables

Allow the default prefixes "a/" and "b/" to be tweaked by the
diff.srcPrefix and diff.dstPrefix configuration variables.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/diff.txt

@@ -111,6 +111,12 @@ diff.mnemonicPrefix::
 diff.noprefix::
 	If set, 'git diff' does not show any source or destination prefix.
 
+diff.srcPrefix::
+	If set, 'git diff' uses this source prefix. Defaults to "a/".
+
+diff.dstPrefix::
+	If set, 'git diff' uses this destination prefix. Defaults to "b/".
+
 diff.relative::
 	If set to 'true', 'git diff' does not show changes outside of the directory
 	and show pathnames relative to the current directory.

Documentation/diff-options.txt

@@ -865,8 +865,9 @@ endif::git-format-patch[]
 
 --default-prefix::
 	Use the default source and destination prefixes ("a/" and "b/").
-	This is usually the default already, but may be used to override
+	This overrides configuration variables such as `diff.noprefix`,
-	config such as `diff.noprefix`.
+	`diff.srcPrefix`, `diff.dstPrefix`, and `diff.mnemonicPrefix`
+	(see `git-config`(1)).
 
 --line-prefix=<prefix>::
 	Prepend an additional prefix to every line of output.

diff.c

@@ -62,6 +62,8 @@ static const char *diff_order_file_cfg;
 int diff_auto_refresh_index = 1;
 static int diff_mnemonic_prefix;
 static int diff_no_prefix;
+static const char *diff_src_prefix = "a/";
+static const char *diff_dst_prefix = "b/";
 static int diff_relative;
 static int diff_stat_name_width;
 static int diff_stat_graph_width;
@@ -408,6 +410,12 @@ int git_diff_ui_config(const char *var, const char *value,
 		diff_no_prefix = git_config_bool(var, value);
 		return 0;
 	}
+	if (!strcmp(var, "diff.srcprefix")) {
+		return git_config_string(&diff_src_prefix, var, value);
+	}
+	if (!strcmp(var, "diff.dstprefix")) {
+		return git_config_string(&diff_dst_prefix, var, value);
+	}
 	if (!strcmp(var, "diff.relative")) {
 		diff_relative = git_config_bool(var, value);
 		return 0;
@@ -3425,8 +3433,8 @@ void diff_set_noprefix(struct diff_options *options)
 
 void diff_set_default_prefix(struct diff_options *options)
 {
-	options->a_prefix = "a/";
+	options->a_prefix = diff_src_prefix;
-	options->b_prefix = "b/";
+	options->b_prefix = diff_dst_prefix;
 }
 
 struct userdiff_driver *get_textconv(struct repository *r,
@@ -5362,6 +5370,8 @@ static int diff_opt_default_prefix(const struct option *opt,
 
 	BUG_ON_OPT_NEG(unset);
 	BUG_ON_OPT_ARG(optarg);
+	diff_src_prefix = "a/";
+	diff_dst_prefix = "b/";
 	diff_set_default_prefix(options);
 	return 0;
 }

t/t4013-diff-various.sh

@@ -663,6 +663,41 @@ test_expect_success 'diff --default-prefix overrides diff.mnemonicprefix' '
 	check_prefix actual a/file0 b/file0
 '
 
+test_expect_success 'diff respects diff.srcprefix' '
+	git -c diff.srcprefix=x/ diff >actual &&
+	check_prefix actual x/file0 b/file0
+'
+
+test_expect_success 'diff respects diff.dstprefix' '
+	git -c diff.dstprefix=y/ diff >actual &&
+	check_prefix actual a/file0 y/file0
+'
+
+test_expect_success 'diff --src-prefix overrides diff.srcprefix' '
+	git -c diff.srcprefix=y/ diff --src-prefix=z/ >actual &&
+	check_prefix actual z/file0 b/file0
+'
+
+test_expect_success 'diff --dst-prefix overrides diff.dstprefix' '
+	git -c diff.dstprefix=y/ diff --dst-prefix=z/ >actual &&
+	check_prefix actual a/file0 z/file0
+'
+
+test_expect_success 'diff.{src,dst}prefix ignored with diff.noprefix' '
+	git -c diff.dstprefix=y/ -c diff.srcprefix=x/ -c diff.noprefix diff >actual &&
+	check_prefix actual file0 file0
+'
+
+test_expect_success 'diff.{src,dst}prefix ignored with diff.mnemonicprefix' '
+	git -c diff.dstprefix=x/ -c diff.srcprefix=y/ -c diff.mnemonicprefix diff >actual &&
+	check_prefix actual i/file0 w/file0
+'
+
+test_expect_success 'diff.{src,dst}prefix ignored with --default-prefix' '
+	git -c diff.dstprefix=x/ -c diff.srcprefix=y/ diff --default-prefix >actual &&
+	check_prefix actual a/file0 b/file0
+'
+
 test_expect_success 'diff --no-renames cannot be abbreviated' '
 	test_expect_code 129 git diff --no-rename >actual 2>error &&
 	test_must_be_empty actual &&