Fix global repository field not being cleared

It is checked for w.r.t. global repository struct
down in the callstack in compatibility layer for MinGW
before being assigned in the function that `free()`'d it.

repository.c

@@ -46,7 +46,7 @@ static void repo_set_commondir(struct repository *repo,
 {
 	struct strbuf sb = STRBUF_INIT;
 
-	free(repo->commondir);
+	FREE_AND_NULL(repo->commondir);
 
 	if (commondir) {
 		repo->different_commondir = 1;