winhttp: retry authentication

If the caller has provided bad authentication, give them another
apportunity to get it right until they give up. This brings WinHTTP in
line with the other transports.

src/transports/winhttp.c

@@ -926,10 +926,11 @@ replay:
 			if (parse_unauthorized_response(s->request, &allowed_types, &t->auth_mechanism) < 0)
 				return -1;
 
-			if (allowed_types &&
-				(!t->cred || 0 == (t->cred->credtype & allowed_types))) {
+			if (allowed_types) {
 				int cred_error = 1;
 
+				git_cred_free(t->cred);
+				t->cred = NULL;
 				/* Start with the user-supplied credential callback, if present */
 				if (t->owner->cred_acquire_cb) {
 					cred_error = t->owner->cred_acquire_cb(&t->cred, t->owner->url,