Adding missing field important to find branch in fork from same owner (#2873)

Fixes: #2872.

Author: r-aramizu

example/commitpr/main.go

@@ -38,6 +38,7 @@ var (
 	sourceRepo    = flag.String("source-repo", "", "Name of repo to create the commit in.")
 	commitMessage = flag.String("commit-message", "", "Content of the commit message.")
 	commitBranch  = flag.String("commit-branch", "", "Name of branch to create the commit in. If it does not already exists, it will be created using the `base-branch` parameter")
+	repoBranch    = flag.String("repo-branch", "", "Name of the repository where the changes in the pull request were made. This field is required for cross-repository pull requests if both repositories are owned by the same organization")
 	baseBranch    = flag.String("base-branch", "master", "Name of branch to create the `commit-branch` from.")
 	prRepoOwner   = flag.String("merge-repo-owner", "", "Name of the owner (user or org) of the repo to create the PR against. If not specified, the value of the `-source-owner` flag will be used.")
 	prRepo        = flag.String("merge-repo", "", "Name of repo to create the PR against. If not specified, the value of the `-source-repo` flag will be used.")
@@ -164,6 +165,7 @@ func createPR() (err error) {
 	newPR := &github.NewPullRequest{
 		Title:               prSubject,
 		Head:                commitBranch,
+		HeadRepo:            repoBranch,
 		Base:                prBranch,
 		Body:                prDescription,
 		MaintainerCanModify: github.Bool(true),

github/github-accessors.go

@@ -244,6 +244,7 @@ func (s *PullRequestsService) GetRaw(ctx context.Context, owner string, repo str
 type NewPullRequest struct {
 	Title               *string `json:"title,omitempty"`
 	Head                *string `json:"head,omitempty"`
+	HeadRepo            *string `json:"head_repo,omitempty"`
 	Base                *string `json:"base,omitempty"`
 	Body                *string `json:"body,omitempty"`
 	Issue               *int    `json:"issue,omitempty"`

github/github-accessors_test.go

@@ -933,6 +933,7 @@ func TestNewPullRequest_Marshal(t *testing.T) {
 	u := &NewPullRequest{
 		Title:               String("eh"),
 		Head:                String("eh"),
+		HeadRepo:            String("eh"),
 		Base:                String("eh"),
 		Body:                String("eh"),
 		Issue:               Int(1),
@@ -943,6 +944,7 @@ func TestNewPullRequest_Marshal(t *testing.T) {
 	want := `{
 		"title": "eh",
 		"head": "eh",
+		"head_repo": "eh",
 		"base": "eh",
 		"body": "eh",
 		"issue": 1,