Fix undefined method one? when pushing to an existing merge request

An untested code path was triggering an Exception because Fixnum
doesn't have `one?` implemented in Rails, while arrays and collections
do.

Closes #45152

app/views/notify/push_to_merge_request_email.html.haml

@@ -7,7 +7,7 @@
   - count = @existing_commits.size
   %ul
     %li
-      - if count.one?
+      - if count == 1
         - commit_id = @existing_commits.first[:short_id]
         = link_to(commit_id, project_commit_url(@merge_request.target_project, commit_id))
       - else

app/views/notify/push_to_merge_request_email.text.haml

@@ -4,7 +4,7 @@
 \
 - if @existing_commits.any?
   - count = @existing_commits.size
-  - commits_id = count.one? ? @existing_commits.first[:short_id] : "#{@existing_commits.first[:short_id]}...#{@existing_commits.last[:short_id]}"
+  - commits_id = count == 1 ? @existing_commits.first[:short_id] : "#{@existing_commits.first[:short_id]}...#{@existing_commits.last[:short_id]}"
   - commits_text = "#{count} commit".pluralize(count)
 
   * #{commits_id} - #{commits_text} from branch `#{@merge_request.target_branch}`

spec/mailers/notify_spec.rb

@@ -390,11 +390,11 @@ describe Notify do
         end
       end
 
-      describe 'that have new commits' do
+      shared_examples 'a push to an existing merge request' do
         let(:push_user) { create(:user) }
 
         subject do
-          described_class.push_to_merge_request_email(recipient.id, merge_request.id, push_user.id, new_commits: merge_request.commits)
+          described_class.push_to_merge_request_email(recipient.id, merge_request.id, push_user.id, new_commits: merge_request.commits, existing_commits: existing_commits)
         end
 
         it_behaves_like 'a multiple recipients email'
@@ -419,6 +419,18 @@ describe Notify do
           end
         end
       end
+
+      describe 'that have new commits' do
+        let(:existing_commits) { [] }
+
+        it_behaves_like 'a push to an existing merge request'
+      end
+
+      describe 'that have new commits on top of an existing one' do
+        let(:existing_commits) { [merge_request.commits.first] }
+
+        it_behaves_like 'a push to an existing merge request'
+      end
     end
 
     context 'for issue notes' do