parse_config_key: use skip_prefix instead of starts_with

This saves us having to repeatedly add in "section_len" (and
also avoids walking over the first part of the string
multiple times for a strlen() and strrchr()).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -2531,11 +2531,10 @@ int parse_config_key(const char *var,
 		     const char **subsection, int *subsection_len,
 		     const char **key)
 {
-	int section_len = strlen(section);
 	const char *dot;
 
 	/* Does it start with "section." ? */
-	if (!starts_with(var, section) || var[section_len] != '.')
+	if (!skip_prefix(var, section, &var) || *var != '.')
 		return -1;
 
 	/*
@@ -2547,12 +2546,12 @@ int parse_config_key(const char *var,
 	*key = dot + 1;
 
 	/* Did we have a subsection at all? */
-	if (dot == var + section_len) {
+	if (dot == var) {
 		*subsection = NULL;
 		*subsection_len = 0;
 	}
 	else {
-		*subsection = var + section_len + 1;
+		*subsection = var + 1;
 		*subsection_len = dot - *subsection;
 	}