Separately query viewerMergeBodyText for GHE compatibility

GHE versions 2.22 and older will not have this GraphQL field. Avoid the
resulting error and have the command proceeed with empty text as the
default.

api/queries_pr.go

@@ -88,8 +88,6 @@ type PullRequest struct {
 	ReactionGroups ReactionGroups
 	Reviews        PullRequestReviews
 	ReviewRequests ReviewRequests
-
-	ViewerMergeBodyText string
 }
 
 type ReviewRequests struct {
@@ -578,7 +576,6 @@ func PullRequestByNumber(client *Client, repo ghrepo.Interface, number int) (*Pu
 				milestone{
 					title
 				}
-				viewerMergeBodyText
 				` + commentsFragment() + `
 				` + reactionGroupsFragment() + `
 			}

pkg/cmd/pr/merge/http.go

@@ -3,6 +3,7 @@ package merge
 import (
 	"context"
 	"net/http"
+	"strings"
 
 	"github.com/cli/cli/internal/ghinstance"
 	"github.com/cli/cli/internal/ghrepo"
@@ -98,3 +99,40 @@ func disableAutoMerge(client *http.Client, repo ghrepo.Interface, prID string) e
 	gql := graphql.NewClient(ghinstance.GraphQLEndpoint(repo.RepoHost()), client)
 	return gql.MutateNamed(context.Background(), "PullRequestAutoMergeDisable", &mutation, variables)
 }
+
+func getMergeText(client *http.Client, repo ghrepo.Interface, prID string, mergeMethod PullRequestMergeMethod) (string, error) {
+	var method githubv4.PullRequestMergeMethod
+	switch mergeMethod {
+	case PullRequestMergeMethodMerge:
+		method = githubv4.PullRequestMergeMethodMerge
+	case PullRequestMergeMethodRebase:
+		method = githubv4.PullRequestMergeMethodRebase
+	case PullRequestMergeMethodSquash:
+		method = githubv4.PullRequestMergeMethodSquash
+	}
+
+	var query struct {
+		Node struct {
+			PullRequest struct {
+				ViewerMergeBodyText string `graphql:"viewerMergeBodyText(mergeType: $method)"`
+			} `graphql:"...on PullRequest"`
+		} `graphql:"node(id: $prID)"`
+	}
+
+	variables := map[string]interface{}{
+		"prID":   githubv4.ID(prID),
+		"method": method,
+	}
+
+	gql := graphql.NewClient(ghinstance.GraphQLEndpoint(repo.RepoHost()), client)
+	err := gql.QueryNamed(context.Background(), "PullRequestMergeText", &query, variables)
+	if err != nil {
+		// Tolerate this API missing in older GitHub Enterprise
+		if strings.Contains(err.Error(), "Field 'viewerMergeBodyText' doesn't exist") {
+			return "", nil
+		}
+		return "", err
+	}
+
+	return query.Node.PullRequest.ViewerMergeBodyText, nil
+}

pkg/cmd/pr/merge/merge.go

@@ -204,11 +204,10 @@ func mergeRun(opts *MergeOptions) error {
 					return err
 				}
 
-				if payload.commitBody == "" {
-					if payload.method == PullRequestMergeMethodMerge {
-						payload.commitBody = pr.Title
-					} else {
-						payload.commitBody = pr.ViewerMergeBodyText
+				if !payload.setCommitBody {
+					payload.commitBody, err = getMergeText(httpClient, baseRepo, pr.ID, payload.method)
+					if err != nil {
+						return err
 					}
 				}
 

pkg/cmd/pr/merge/merge_test.go

@@ -703,6 +703,12 @@ func TestPRMerge_interactiveSquashEditCommitMsg(t *testing.T) {
 			"rebaseMergeAllowed": true,
 			"squashMergeAllowed": true
 		} } }`))
+	http.Register(
+		httpmock.GraphQL(`query PullRequestMergeText\b`),
+		httpmock.StringResponse(`
+		{ "data": { "node": {
+			"viewerMergeBodyText": ""
+		} } }`))
 	http.Register(
 		httpmock.GraphQL(`mutation PullRequestMerge\b`),
 		httpmock.GraphQLMutation(`{}`, func(input map[string]interface{}) {