Add list branch function for github util package (#1400)

* Add list branch function for github util package

* Fix unit test

test/ghutil/client.go

@@ -61,6 +61,7 @@ type GithubOperations interface {
 	ListCommits(org, repo string, ID int) ([]*github.RepositoryCommit, error)
 	ListFiles(org, repo string, ID int) ([]*github.CommitFile, error)
 	CreatePullRequest(org, repo, head, base, title, body string) (*github.PullRequest, error)
+	ListBranches(org, repo string) ([]*github.Branch, error)
 }
 
 // GithubClient provides methods to perform github operations

test/ghutil/fakeghutil/fakeghutil.go

@@ -36,6 +36,7 @@ type FakeGithubClient struct {
 	PullRequests map[string]map[int]*github.PullRequest // map of repo: map of PullRequest Number: pullrequests
 	PRCommits    map[int][]*github.RepositoryCommit     // map of PR number: slice of commits
 	CommitFiles  map[string][]*github.CommitFile        // map of commit SHA: slice of files
+	Branches     map[string][]*github.Branch            // map of repo: branches
 
 	NextNumber int    // number to be assigned to next newly created issue/comment
 	BaseURL    string // base URL of Github
@@ -330,6 +331,15 @@ func (fgc *FakeGithubClient) CreatePullRequest(org, repo, head, base, title, bod
 	return PR, nil
 }
 
+// ListBranches lists branchs for given repo
+func (fgc *FakeGithubClient) ListBranches(org, repo string) ([]*github.Branch, error) {
+	var branches []*github.Branch
+	for _, b := range fgc.Branches {
+		branches = append(branches, b...)
+	}
+	return branches, nil
+}
+
 // AddFileToCommit adds file to commit
 // This is complementary of mocking CreatePullRequest, so that newly created pull request can have files
 func (fgc *FakeGithubClient) AddFileToCommit(org, repo, SHA, filename, patch string) error {

test/ghutil/repo.go

@@ -0,0 +1,49 @@
+/*
+Copyright 2020 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.
+*/
+
+// repo.go provides generic functions related to Repo
+
+package ghutil
+
+import (
+	"fmt"
+
+	"github.com/google/go-github/v27/github"
+)
+
+// ListBranches lists branchs for given repo
+func (gc *GithubClient) ListBranches(org, repo string) ([]*github.Branch, error) {
+	genericList, err := gc.depaginate(
+		fmt.Sprintf("listing Pull request from org %q and base %q", org, repo),
+		maxRetryCount,
+		&github.ListOptions{},
+		func() ([]interface{}, *github.Response, error) {
+			page, resp, err := gc.Client.Repositories.ListBranches(ctx, org, repo, nil)
+			var interfaceList []interface{}
+			if nil == err {
+				for _, PR := range page {
+					interfaceList = append(interfaceList, PR)
+				}
+			}
+			return interfaceList, resp, err
+		},
+	)
+	res := make([]*github.Branch, len(genericList))
+	for i, elem := range genericList {
+		res[i] = elem.(*github.Branch)
+	}
+	return res, err
+}