diff options: Introduce --ignore-submodules

The new option --ignore-submodules can now be used to ignore changes in
submodules.

Why?  Sometimes it is not interesting when a submodule changed.

For example, when reordering some commits in the superproject, a dirty
submodule is usually totally uninteresting.  So we will use this option
in git-rebase to test for a dirty working tree.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -228,6 +228,9 @@ endif::git-format-patch[]
 --no-ext-diff::
 	Disallow external diff drivers.
 
+--ignore-submodules::
+	Ignore changes to submodules in the diff generation.
+
 --src-prefix=<prefix>::
 	Show the given source prefix instead of "a/".
 

diff.c

@@ -2496,6 +2496,8 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 		DIFF_OPT_SET(options, ALLOW_EXTERNAL);
 	else if (!strcmp(arg, "--no-ext-diff"))
 		DIFF_OPT_CLR(options, ALLOW_EXTERNAL);
+	else if (!strcmp(arg, "--ignore-submodules"))
+		DIFF_OPT_SET(options, IGNORE_SUBMODULES);
 
 	/* misc options */
 	else if (!strcmp(arg, "-z"))
@@ -3355,6 +3357,9 @@ void diff_addremove(struct diff_options *options,
 	char concatpath[PATH_MAX];
 	struct diff_filespec *one, *two;
 
+	if (DIFF_OPT_TST(options, IGNORE_SUBMODULES) && S_ISGITLINK(mode))
+		return;
+
 	/* This may look odd, but it is a preparation for
 	 * feeding "there are unchanged files which should
 	 * not produce diffs, but when you are doing copy
@@ -3399,6 +3404,10 @@ void diff_change(struct diff_options *options,
 	char concatpath[PATH_MAX];
 	struct diff_filespec *one, *two;
 
+	if (DIFF_OPT_TST(options, IGNORE_SUBMODULES) && S_ISGITLINK(old_mode)
+			&& S_ISGITLINK(new_mode))
+		return;
+
 	if (DIFF_OPT_TST(options, REVERSE_DIFF)) {
 		unsigned tmp;
 		const unsigned char *tmp_c;

diff.h

@@ -63,6 +63,7 @@ typedef void (*diff_format_fn_t)(struct diff_queue_struct *q,
 #define DIFF_OPT_REVERSE_DIFF        (1 << 15)
 #define DIFF_OPT_CHECK_FAILED        (1 << 16)
 #define DIFF_OPT_RELATIVE_NAME       (1 << 17)
+#define DIFF_OPT_IGNORE_SUBMODULES   (1 << 18)
 #define DIFF_OPT_TST(opts, flag)    ((opts)->flags & DIFF_OPT_##flag)
 #define DIFF_OPT_SET(opts, flag)    ((opts)->flags |= DIFF_OPT_##flag)
 #define DIFF_OPT_CLR(opts, flag)    ((opts)->flags &= ~DIFF_OPT_##flag)