Add specs for single-letter external tracker refs

spec/lib/banzai/filter/external_issue_reference_filter_spec.rb

@@ -101,16 +101,25 @@ describe Banzai::Filter::ExternalIssueReferenceFilter do
 
   context "redmine project" do
     let(:project) { create(:redmine_project) }
+
+    before do
+      project.update!(issues_enabled: false)
+    end
+
+    context "with a hash prefix" do
       let(:issue) { ExternalIssue.new("#123", project) }
       let(:reference) { issue.to_reference }
 
-    before do
-      project.issues_enabled = false
-      project.save!
+      it_behaves_like "external issue tracker"
     end
 
+    context "with a single-letter prefix" do
+      let(:issue) { ExternalIssue.new("T-123", project) }
+      let(:reference) { issue.to_reference }
+
       it_behaves_like "external issue tracker"
     end
+  end
 
   context "jira project" do
     let(:project) { create(:jira_project) }
@@ -122,6 +131,15 @@ describe Banzai::Filter::ExternalIssueReferenceFilter do
       it_behaves_like "external issue tracker"
     end
 
+    context "with a single-letter prefix" do
+      let(:issue) { ExternalIssue.new("J-123", project) }
+
+      it "ignores reference" do
+        exp = act = "Issue #{reference}"
+        expect(filter(act).to_html).to eq exp
+      end
+    end
+
     context "with wrong markdown" do
       let(:issue) { ExternalIssue.new("#123", project) }