Introduce `fake` Git implementation

At present it only implements a fake commit, which for example can be
used to test commit verification logic.

Signed-off-by: Hidde Beydals <hello@hidde.co>

pkg/git/fake/commit.go

@@ -0,0 +1,46 @@
+/*
+Copyright 2021 The Flux authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package fake
+
+import (
+	"errors"
+
+	corev1 "k8s.io/api/core/v1"
+)
+
+type Commit struct {
+	valid bool
+	hash  string
+}
+
+func NewCommit(valid bool, hash string) Commit {
+	return Commit{
+		valid: valid,
+		hash:  hash,
+	}
+}
+
+func (c Commit) Verify(secret corev1.Secret) error {
+	if !c.valid {
+		return errors.New("invalid signature")
+	}
+	return nil
+}
+
+func (c Commit) Hash() string {
+	return c.hash
+}