Add support for git commit sha

copying solution from tektoncd pipeline: tektoncd/pipeline#555
Credit to @dibbles on the original PR

cmd/git-init/main.go

@@ -42,15 +42,17 @@ func run(logger *zap.SugaredLogger, cmd string, args ...string) {
 	}
 }
 
-func runOrFail(logger *zap.SugaredLogger, cmd string, args ...string) {
+func runOrFail(logger *zap.SugaredLogger, cmd string, args ...string) error {
 	c := exec.Command(cmd, args...)
 	var output bytes.Buffer
 	c.Stderr = &output
 	c.Stdout = &output
 
 	if err := c.Run(); err != nil {
-		logger.Fatalf("Unexpected error running %v %v: %v\n%v", cmd, args, err, output.String())
+		logger.Errorf("Unexpected error running %v %v: %v\n%v", cmd, args, err, output.String())
+		return err
 	}
+	return nil
 }
 
 func main() {
@@ -86,8 +88,15 @@ func main() {
 	}
 
 	run(logger, "git", "remote", "add", "origin", *url)
-	runOrFail(logger, "git", "fetch", "--depth=1", "--recurse-submodules=yes", "origin", *revision)
-	runOrFail(logger, "git", "reset", "--hard", "FETCH_HEAD")
+	err = runOrFail(logger, "git", "fetch", "--depth=1", "--recurse-submodules=yes", "origin", *revision)
+	if err != nil {
+		// Fetch can fail if an old commitid was used so try git pull, performing regardless of error
+		// as no guarantee that the same error is returned by all git servers gitlab, github etc...
+		run(logger, "git", "pull", "--recurse-submodules=yes", "origin")
+		runOrFail(logger, "git", "checkout", *revision)
+	} else {
+		runOrFail(logger, "git", "reset", "--hard", "FETCH_HEAD")
+	}
 
 	logger.Infof("Successfully cloned %q @ %q in path %q", *url, *revision, dir)
 }

test/git-source/1-commitid.yaml

@@ -0,0 +1,26 @@
+# Copyright 2018 The Knative 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.
+apiVersion: build.knative.dev/v1alpha1
+kind: Build
+metadata:
+  name: test-git-commitid
+  labels:
+    expect: succeeded
+spec:
+  source:
+    git:
+      url: https://github.com/bazelbuild/rules_docker
+      revision: 4025a58c156a4ba0fd3606df50051423043495bf
+  template:
+    name: dump-workspace-template