diff: introduce diff.submodule configuration variable

Introduce a diff.submodule configuration variable corresponding to the
'--submodule' command-line option of 'git diff'.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-config.txt

@@ -107,6 +107,13 @@ diff.suppressBlankEmpty::
 	A boolean to inhibit the standard behavior of printing a space
 	before each empty output line. Defaults to false.
 
+diff.submodule::
+	Specify the format in which differences in submodules are
+	shown.  The "log" format lists the commits in the range like
+	linkgit:git-submodule[1] `summary` does.  The "short" format
+	format just shows the names of the commits at the beginning
+	and end of the range.  Defaults to short.
+
 diff.wordRegex::
 	A POSIX Extended Regular Expression used to determine what is a "word"
 	when performing word-by-word difference calculations.  Character

Documentation/diff-options.txt

@@ -170,7 +170,8 @@ any of those replacements occurred.
 	the commits in the range like linkgit:git-submodule[1] `summary` does.
 	Omitting the `--submodule` option or specifying `--submodule=short`,
 	uses the 'short' format. This format just shows the names of the commits
-	at the beginning and end of the range.
+	at the beginning and end of the range.  Can be tweaked via the
+	`diff.submodule` configuration variable.
 
 --color[=<when>]::
 	Show colored diff.

diff.c

@@ -121,6 +121,17 @@ static int parse_dirstat_params(struct diff_options *options, const char *params
 	return ret;
 }
 
+static int parse_submodule_params(struct diff_options *options, const char *value)
+{
+	if (!strcmp(value, "log"))
+		DIFF_OPT_SET(options, SUBMODULE_LOG);
+	else if (!strcmp(value, "short"))
+		DIFF_OPT_CLR(options, SUBMODULE_LOG);
+	else
+		return -1;
+	return 0;
+}
+
 static int git_config_rename(const char *var, const char *value)
 {
 	if (!value)
@@ -176,6 +187,13 @@ int git_diff_ui_config(const char *var, const char *value, void *cb)
 	if (!strcmp(var, "diff.ignoresubmodules"))
 		handle_ignore_submodules_arg(&default_diff_options, value);
 
+	if (!strcmp(var, "diff.submodule")) {
+		if (parse_submodule_params(&default_diff_options, value))
+			warning(_("Unknown value for 'diff.submodule' config variable: '%s'"),
+				value);
+		return 0;
+	}
+
 	if (git_color_config(var, value, cb) < 0)
 		return -1;
 
@@ -3473,6 +3491,14 @@ static int parse_dirstat_opt(struct diff_options *options, const char *params)
 	return 1;
 }
 
+static int parse_submodule_opt(struct diff_options *options, const char *value)
+{
+	if (parse_submodule_params(options, value))
+		die(_("Failed to parse --submodule option parameter: '%s'"),
+			value);
+	return 1;
+}
+
 int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 {
 	const char *arg = av[0];
@@ -3653,10 +3679,8 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 		handle_ignore_submodules_arg(options, arg + 20);
 	} else if (!strcmp(arg, "--submodule"))
 		DIFF_OPT_SET(options, SUBMODULE_LOG);
-	else if (!prefixcmp(arg, "--submodule=")) {
-		if (!strcmp(arg + 12, "log"))
-			DIFF_OPT_SET(options, SUBMODULE_LOG);
-	}
+	else if (!prefixcmp(arg, "--submodule="))
+		return parse_submodule_opt(options, arg + 12);
 
 	/* misc options */
 	else if (!strcmp(arg, "-z"))

t/t4041-diff-submodule-option.sh

@@ -33,6 +33,7 @@ test_create_repo sm1 &&
 add_file . foo >/dev/null
 
 head1=$(add_file sm1 foo1 foo2)
+fullhead1=$(cd sm1; git rev-list --max-count=1 $head1)
 
 test_expect_success 'added submodule' "
 	git add sm1 &&
@@ -43,6 +44,33 @@ EOF
 	test_cmp expected actual
 "
 
+test_expect_success 'added submodule, set diff.submodule' "
+	git config diff.submodule log &&
+	git add sm1 &&
+	git diff --cached >actual &&
+	cat >expected <<-EOF &&
+Submodule sm1 0000000...$head1 (new submodule)
+EOF
+	git config --unset diff.submodule &&
+	test_cmp expected actual
+"
+
+test_expect_success '--submodule=short overrides diff.submodule' "
+	test_config diff.submodule log &&
+	git add sm1 &&
+	git diff --submodule=short --cached >actual &&
+	cat >expected <<-EOF &&
+diff --git a/sm1 b/sm1
+new file mode 160000
+index 0000000..a2c4dab
+--- /dev/null
++++ b/sm1
+@@ -0,0 +1 @@
++Subproject commit $fullhead1
+EOF
+	test_cmp expected actual
+"
+
 commit_file sm1 &&
 head2=$(add_file sm1 foo3)
 
@@ -73,7 +101,6 @@ EOF
 	test_cmp expected actual
 "
 
-fullhead1=$(cd sm1; git rev-list --max-count=1 $head1)
 fullhead2=$(cd sm1; git rev-list --max-count=1 $head2)
 test_expect_success 'modified submodule(forward) --submodule=short' "
 	git diff --submodule=short >actual &&