read-cache.c: replace `git_config()` with `git_config_get_*()` family

Use `git_config_get_*()` family instead of `git_config()` to take
advantage of the config-set API which provides a cleaner control flow.

Use an intermediate value, as `version` can not be used directly in
git_config_get_int() due to incompatible type.

Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -1238,24 +1238,16 @@ static struct cache_entry *refresh_cache_entry(struct cache_entry *ce,
 
 #define INDEX_FORMAT_DEFAULT 3
 
-static int index_format_config(const char *var, const char *value, void *cb)
-{
-	unsigned int *version = cb;
-	if (!strcmp(var, "index.version")) {
-		*version = git_config_int(var, value);
-		return 0;
-	}
-	return 1;
-}
-
 static unsigned int get_index_format_default(void)
 {
 	char *envversion = getenv("GIT_INDEX_VERSION");
 	char *endp;
+	int value;
 	unsigned int version = INDEX_FORMAT_DEFAULT;
 
 	if (!envversion) {
-		git_config(index_format_config, &version);
+		if (!git_config_get_int("index.version", &value))
+			version = value;
 		if (version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < version) {
 			warning(_("index.version set, but the value is invalid.\n"
 				  "Using version %i"), INDEX_FORMAT_DEFAULT);