[PATCH v3] diff: add diff.srcPrefix and diff.dstPrefix configuration variables

[Peter Hutterer <peter.hutterer@who-t.net>]

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>
---
Changes to v2;
- doc: change to camelcase diff.srcPrefix/diff.dstPrefix for
  consistency with diff.mnemonicPrefix and most other options
- git diff --default-prefix forces a/ and b/ regardless of configured
  prefix, see the 'diff_opt_default_prefix' hunk in the patch below.

The latter may be slightly controversial but: there are scripts out
there that rely on the a/ and b/ prefix (came across one last night).
With a custom prefix those scripts will break, having an option that
forces the a/ and b/ prefix helps. Plus the man page explicitly says:
  Use the default source and destination prefixes ("a/" and "b/").
So let's stick with that behaviour then.

 Documentation/config/diff.txt |  6 ++++++
 diff.c                        | 14 ++++++++++++--
 t/t4013-diff-various.sh       | 35 +++++++++++++++++++++++++++++++++++
 3 files changed, 53 insertions(+), 2 deletions(-)

diff --git a/Documentation/config/diff.txt b/Documentation/config/diff.txt
index 6c7e09a1ef5e..afc23d7723b6 100644
--- a/Documentation/config/diff.txt
+++ b/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.
diff --git a/diff.c b/diff.c
index e50def45383e..108c1875775d 100644
--- a/diff.c
+++ b/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->b_prefix = "b/";
+	options->a_prefix = diff_src_prefix;
+	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;
 }
diff --git a/t/t4013-diff-various.sh b/t/t4013-diff-various.sh
index 1e3b2dbea484..e75f9f7d4cb2 100755
--- a/t/t4013-diff-various.sh
+++ b/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=z/ 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/dstprefix 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/dstprefix 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/dstprefix 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 &&
-- 
2.44.0