Let the server choose the commit subject for squashed merge

For single-commit PRs, the commit subject will be the subject of the
head commit and the PR number. For multi-commit PRs, the commit subject
will be the PR title and PR number. Instead of trying to replicate this
logic client-side, omit the `commitHeadline` param and let the server
apply defaults appropriately.

Reverts https://github.com/cli/cli/pull/1627

pkg/cmd/pr/merge/merge.go

@@ -229,11 +229,6 @@ func mergeRun(opts *MergeOptions) error {
 			}
 		}
 
-		if !payload.setCommitSubject && payload.method == PullRequestMergeMethodSquash {
-			payload.commitSubject = fmt.Sprintf("%s (#%d)", pr.Title, pr.Number)
-			payload.setCommitSubject = true
-		}
-
 		err = mergePullRequest(httpClient, payload)
 		if err != nil {
 			return err

pkg/cmd/pr/merge/merge_test.go

@@ -500,7 +500,7 @@ func TestPrMerge_squash(t *testing.T) {
 		httpmock.GraphQLMutation(`{}`, func(input map[string]interface{}) {
 			assert.Equal(t, "THE-ID", input["pullRequestId"].(string))
 			assert.Equal(t, "SQUASH", input["mergeMethod"].(string))
-			assert.Equal(t, "The title of the PR (#3)", input["commitHeadline"].(string))
+			assert.NotContains(t, input, "commitHeadline")
 		}))
 
 	_, cmdTeardown := run.Stub()