use strchrnul() in place of strchr() and strlen()

Avoid scanning strings twice, once with strchr() and then with
strlen(), by using strchrnul().

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Rohit Mani <rohit.mani@outlook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/parse-options.c b/parse-options.c
index 7b8d3fa..a5fa0b8 100644
--- a/parse-options.c
+++ b/parse-options.c
@@ -223,13 +223,10 @@
                           const struct option *options)
 {
 	const struct option *all_opts = options;
-	const char *arg_end = strchr(arg, '=');
+	const char *arg_end = strchrnul(arg, '=');
 	const struct option *abbrev_option = NULL, *ambiguous_option = NULL;
 	int abbrev_flags = 0, ambiguous_flags = 0;
 
-	if (!arg_end)
-		arg_end = arg + strlen(arg);
-
 	for (; options->type != OPTION_END; options++) {
 		const char *rest, *long_name = options->long_name;
 		int flags = 0, opt_flags = 0;