api: gracefully handle already deleted remote refs

If a GitHub repo is configured to automatically delete branches after PR
is merged, `gh pr merge` fails with error like:

failed to delete remote branch: ... (422): 'Reference does not exist'

Gracefully handle such case and don't report the error.

Signed-off-by: Pavel Borzenkov <pavel.borzenkov@gmail.com>

api/queries_pr.go

@@ -1015,7 +1015,16 @@ func BranchDeleteRemote(client *Client, repo ghrepo.Interface, branch string) er
 		NodeID string `json:"node_id"`
 	}
 	path := fmt.Sprintf("repos/%s/%s/git/refs/heads/%s", repo.RepoOwner(), repo.RepoName(), branch)
-	return client.REST("DELETE", path, nil, &response)
+	err := client.REST("DELETE", path, nil, &response)
+	if err != nil {
+		var httpErr HTTPError
+		// The ref might have already been deleted by GitHub
+		if !errors.As(err, &httpErr) || httpErr.Code != 422 {
+			return err
+		}
+	}
+
+	return nil
 }
 
 func min(a, b int) int {

api/queries_pr_test.go

@@ -0,0 +1,42 @@
+package api
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/cli/cli/internal/ghrepo"
+	"github.com/cli/cli/pkg/httpmock"
+)
+
+func TestBranchDeleteRemote(t *testing.T) {
+	var tests = []struct {
+		name        string
+		code        int
+		body        string
+		expectError bool
+	}{
+		{name: "success", code: 204, body: "", expectError: false},
+		{name: "error", code: 500, body: `{"message": "oh no"}`, expectError: true},
+		{
+			name:        "already_deleted",
+			code:        422,
+			body:        `{"message": "Reference does not exist"}`,
+			expectError: false,
+		},
+	}
+
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			http := &httpmock.Registry{}
+			client := NewClient(ReplaceTripper(http))
+
+			http.StubResponse(tc.code, bytes.NewBufferString(tc.body))
+			repo, _ := ghrepo.FromFullName("OWNER/REPO")
+			err := BranchDeleteRemote(client, repo, "branch")
+			if isError := err != nil; isError != tc.expectError {
+				t.Fatalf("unexpected result: %v", err)
+			}
+		})
+	}
+}