Add the 'diff.ignoreSubmodules' config setting

When you have a lot of submodules checked out, the time penalty to check
for dirty submodules can easily imply a multiplication of the total time
by the factor 20. This makes the difference between almost instantaneous
(< 2 seconds) and unbearably slow (> 50 seconds) here, since the disk
caches are constantly overloaded.

To this end, the submodule.*.ignore config option was introduced, but it
is per-submodule.

This commit introduces a global config setting to set a default
(porcelain) value for the --ignore-submodules option, keeping the
default at 'none'. It can be overridden by the submodule.*.ignore
setting and by the --ignore-submodules option.

Incidentally, this commit fixes an issue with the overriding logic:
multiple --ignore-submodules options would not clear the previously
set flags.

While at it, fix a typo in the documentation for submodule.*.ignore.

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

Documentation/config.txt

@@ -826,6 +826,11 @@ diff.renames::
 	will enable basic rename detection.  If set to "copies" or
 	"copy", it will detect copies, as well.
 
+diff.ignoreSubmodules::
+	Sets the default value of --ignore-submodules. Note that this
+	affects only 'git diff' Porcelain, and not lower level 'diff'
+	commands such as 'git diff-files'.
+
 diff.suppressBlankEmpty::
 	A boolean to inhibit the standard behavior of printing a space
 	before each empty output line. Defaults to false.
@@ -1754,7 +1759,7 @@ submodule.<name>.ignore::
 	submodules that have untracked files in their work tree as changed.
 	This setting overrides any setting made in .gitmodules for this submodule,
 	both settings can be overriden on the command line by using the
-	"--ignore-submodule" option.
+	"--ignore-submodules" option.
 
 tar.umask::
 	This variable can be used to restrict the permission bits of

diff.c

@@ -31,6 +31,7 @@ static const char *external_diff_cmd_cfg;
 int diff_auto_refresh_index = 1;
 static int diff_mnemonic_prefix;
 static int diff_no_prefix;
+static struct diff_options default_diff_options;
 
 static char diff_colors[][COLOR_MAXLEN] = {
 	GIT_COLOR_RESET,
@@ -107,6 +108,9 @@ int git_diff_ui_config(const char *var, const char *value, void *cb)
 	if (!strcmp(var, "diff.wordregex"))
 		return git_config_string(&diff_word_regex_cfg, var, value);
 
+	if (!strcmp(var, "diff.ignoresubmodules"))
+		handle_ignore_submodules_arg(&default_diff_options, value);
+
 	return git_diff_basic_config(var, value, cb);
 }
 
@@ -2816,7 +2820,7 @@ static void run_checkdiff(struct diff_filepair *p, struct diff_options *o)
 
 void diff_setup(struct diff_options *options)
 {
-	memset(options, 0, sizeof(*options));
+	memcpy(options, &default_diff_options, sizeof(*options));
 	memset(&diff_queued_diff, 0, sizeof(diff_queued_diff));
 
 	options->file = stdout;

submodule.c

@@ -123,6 +123,10 @@ int parse_submodule_config_option(const char *var, const char *value)
 void handle_ignore_submodules_arg(struct diff_options *diffopt,
 				  const char *arg)
 {
+	DIFF_OPT_CLR(diffopt, IGNORE_SUBMODULES);
+	DIFF_OPT_CLR(diffopt, IGNORE_UNTRACKED_IN_SUBMODULES);
+	DIFF_OPT_CLR(diffopt, IGNORE_DIRTY_SUBMODULES);
+
 	if (!strcmp(arg, "all"))
 		DIFF_OPT_SET(diffopt, IGNORE_SUBMODULES);
 	else if (!strcmp(arg, "untracked"))