checkout: respect diff.ignoreSubmodules setting

When 'git checkout' reports uncommitted changes, it also does so for
submodules.

The default mode is now to look really hard into submodules, not only
for different commits, but also for modified files. Since this can be
pretty expensive when there are a lot (and large) submodules, there is
the diff.ignoreSubmodules option.

Let's respect that setting when 'git checkout' reports the uncommitted
changes, since it does nothing else than a 'git diff --name-status'.

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

builtin/checkout.c

@@ -18,6 +18,7 @@
 #include "xdiff-interface.h"
 #include "ll-merge.h"
 #include "resolve-undo.h"
+#include "submodule.h"
 
 static const char * const checkout_usage[] = {
 	"git checkout [options] <branch>",
@@ -36,6 +37,7 @@ struct checkout_opts {
 	const char *new_orphan_branch;
 	int new_branch_log;
 	enum branch_track track;
+	struct diff_options diff_options;
 };
 
 static int post_checkout_hook(struct commit *old, struct commit *new,
@@ -274,12 +276,13 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 	return errs;
 }
 
-static void show_local_changes(struct object *head)
+static void show_local_changes(struct object *head, struct diff_options *opts)
 {
 	struct rev_info rev;
 	/* I think we want full paths, even if we're in a subdirectory. */
 	init_revisions(&rev, NULL);
 	rev.abbrev = 0;
+	rev.diffopt.flags = opts->flags;
 	rev.diffopt.output_format |= DIFF_FORMAT_NAME_STATUS;
 	if (diff_setup_done(&rev.diffopt) < 0)
 		die("diff_setup_done failed");
@@ -456,7 +459,7 @@ static int merge_working_tree(struct checkout_opts *opts,
 		die("unable to write new index file");
 
 	if (!opts->force && !opts->quiet)
-		show_local_changes(&new->commit->object);
+		show_local_changes(&new->commit->object, &opts->diff_options);
 
 	return 0;
 }
@@ -600,7 +603,12 @@ static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
 
 static int git_checkout_config(const char *var, const char *value, void *cb)
 {
-	return git_xmerge_config(var, value, cb);
+	if (!strcmp(var, "diff.ignoresubmodules")) {
+		struct checkout_opts *opts = cb;
+		handle_ignore_submodules_arg(&opts->diff_options, value);
+		return 0;
+	}
+	return git_xmerge_config(var, value, NULL);
 }
 
 static int interactive_checkout(const char *revision, const char **pathspec,
@@ -681,7 +689,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	memset(&opts, 0, sizeof(opts));
 	memset(&new, 0, sizeof(new));
 
-	git_config(git_checkout_config, NULL);
+	git_config(git_checkout_config, &opts);
 
 	opts.track = BRANCH_TRACK_UNSPECIFIED;