parseopt: values of pathname type can be prefixed with :(optional)

In the previous step, we introduced an optional filename that can be
given to a configuration variable, and nullify the fact that such a
configuration setting even existed if the named path is missing or
empty.

Let's do the same for command line options that name a pathname.

Signed-off-by: Junio C Hamano <[e-mail]>
Signed-off-by: Taylor Blau <[e-mail]>
Signed-off-by: D. Ben Knoble <[e-mail]>
Signed-off-by: Junio C Hamano <[e-mail]>

Documentation/gitcli.adoc

···
 $ git describe --abbrev 10 HEAD  # NOT WHAT YOU MEANT
 ----------------------------
 
+
+Magic filename options
+~~~~~~~~~~~~~~~~~~~~~~
+Options that take a filename allow a prefix `:(optional)`. For example:
+
+----------------------------
+git commit -F :(optional)COMMIT_EDITMSG
+# if COMMIT_EDITMSG does not exist, equivalent to
+git commit
+----------------------------
+
+Like with configuration values, if the named file is missing Git behaves as if
+the option was not given at all. See "Values" in linkgit:git-config[1].
+
 NOTES ON FREQUENTLY CONFUSED OPTIONS
 ------------------------------------
 

parse-options.c

···
 {
 	const char *arg;
 	const int unset = flags & OPT_UNSET;
-	int err;
 
 	if (unset && p->opt)
 		return error(_("%s takes no value"), optname(opt, flags));
···
 	case OPTION_FILENAME:
 	{
 		const char *value;
-
-		FREE_AND_NULL(*(char **)opt->value);
-
-		err = 0;
+		int is_optional;
 
 		if (unset)
 			value = NULL;
 		else if (opt->flags & PARSE_OPT_OPTARG && !p->opt)
-			value = (const char *) opt->defval;
-		else
-			err = get_arg(p, opt, flags, &value);
+			value = (char *)opt->defval;
+		else {
+			int err = get_arg(p, opt, flags, &value);
+			if (err)
+				return err;
+		}
+		if (!value)
+			return 0;
 
-		if (!err)
-			*(char **)opt->value = fix_filename(p->prefix, value);
-		return err;
+		is_optional = skip_prefix(value, ":(optional)", &value);
+		if (!value)
+			is_optional = 0;
+		value = fix_filename(p->prefix, value);
+		if (is_optional && is_empty_or_missing_file(value)) {
+			free((char *)value);
+		} else {
+			FREE_AND_NULL(*(char **)opt->value);
+			*(const char **)opt->value = value;
+		}
+		return 0;
 	}
 	case OPTION_CALLBACK:
 	{

t/t7500-commit-template-squash-signoff.sh

···
 	)
 '
 
+test_expect_success 'nonexistent optional template file on command line' '
+	echo changes >> foo &&
+	git add foo &&
+	(
+		GIT_EDITOR="echo hello >\"\$1\"" &&
+		export GIT_EDITOR &&
+		git commit --template ":(optional)$PWD/notexist"
+	)
+'
+
 test_expect_success 'nonexistent template file in config should return error' '
 	test_config commit.template "$PWD"/notexist &&
 	(