remote: add a test to see that we're calling the credentials callback

This does get called when we use the libgit2 stack, but is not yet something the
Go one does.

remote_test.go

@@ -184,3 +184,26 @@ func TestRemotePrune(t *testing.T) {
 		t.Fatal("Expected error getting a pruned reference")
 	}
 }
+
+func TestRemoteCredentialsCalled(t *testing.T) {
+	t.Parallel()
+
+	repo := createTestRepo(t)
+	defer cleanupTestRepo(t, repo)
+
+	remote, err := repo.Remotes.CreateAnonymous("https://github.com/libgit2/non-existent")
+	checkFatal(t, err)
+
+	fetchOpts := FetchOptions{
+		RemoteCallbacks: RemoteCallbacks{
+			CredentialsCallback: func(url, username string, allowedTypes CredType) (ErrorCode, *Cred) {
+				return ErrUser, nil
+			},
+		},
+	}
+
+	err = remote.Fetch(nil, &fetchOpts, "fetch")
+	if !IsErrorCode(err, ErrUser) {
+		t.Fatal("Bad Fetch return, expected ErrUser, got", err)
+	}
+}