pr merge: fix merge queue API access for PAT consumers

The github.com API right now doesn't seem to make merge queue APIs
available to people who use PAT to authenticate gh requests. This
switches the conditional request strategy to also do feature-detection
for dotcom (instead of just for Enterprise).

internal/featuredetection/feature_detection.go

@@ -72,9 +72,12 @@ func (d *detector) IssueFeatures() (IssueFeatures, error) {
 }
 
 func (d *detector) PullRequestFeatures() (PullRequestFeatures, error) {
-	if !ghinstance.IsEnterprise(d.host) {
-		return allPullRequestFeatures, nil
-	}
+	// TODO: reinstate the short-circuit once the APIs are fully available on github.com
+	// https://github.com/cli/cli/issues/5778
+	//
+	// if !ghinstance.IsEnterprise(d.host) {
+	// 	return allPullRequestFeatures, nil
+	// }
 
 	features := PullRequestFeatures{
 		ReviewDecision:       true,

internal/featuredetection/feature_detection_test.go

@@ -20,6 +20,14 @@ func TestPullRequestFeatures(t *testing.T) {
 		{
 			name:     "github.com",
 			hostname: "github.com",
+			queryResponse: map[string]string{
+				`query PullRequest_fields\b`: heredoc.Doc(`
+					{ "data": { "PullRequest": { "fields": [
+						{"name": "isInMergeQueue"},
+						{"name": "isMergeQueueEnabled"}
+					] } } }
+				`),
+			},
 			wantFeatures: PullRequestFeatures{
 				ReviewDecision:       true,
 				StatusCheckRollup:    true,
@@ -28,6 +36,23 @@ func TestPullRequestFeatures(t *testing.T) {
 			},
 			wantErr: false,
 		},
+		{
+			name:     "github.com with no merge queue",
+			hostname: "github.com",
+			queryResponse: map[string]string{
+				`query PullRequest_fields\b`: heredoc.Doc(`
+					{ "data": { "PullRequest": { "fields": [
+					] } } }
+				`),
+			},
+			wantFeatures: PullRequestFeatures{
+				ReviewDecision:       true,
+				StatusCheckRollup:    true,
+				BranchProtectionRule: true,
+				MergeQueue:           false,
+			},
+			wantErr: false,
+		},
 		{
 			name:     "GHE",
 			hostname: "git.my.org",