Add ExternalIssue base model to make issue referencing more robust for external issue trackers.

app/models/concerns/mentionable.rb

@@ -67,9 +67,10 @@ module Mentionable
     return [] if text.blank?
     ext = Gitlab::ReferenceExtractor.new
     ext.analyze(text, p)
-    (ext.issues_for +
-     ext.merge_requests_for +
-     ext.commits_for).uniq - [local_reference]
+
+    (ext.issues_for(p)  +
+     ext.merge_requests_for(p) +
+     ext.commits_for(p)).uniq - [local_reference]
   end
 
   # Create a cross-reference Note for each GFM reference to another Mentionable found in +mentionable_text+.

app/models/external_issue.rb

+class ExternalIssue
+  def initialize(issue_identifier, project)
+    @issue_identifier, @project = issue_identifier, project
+  end
+
+  def to_s
+    @issue_identifier.to_s
+  end
+
+  def id
+    @issue_identifier.to_s
+  end
+
+  def iid
+    @issue_identifier.to_s
+  end
+
+  def ==(other)
+    other.is_a?(self.class) && (to_s == other.to_s)
+  end
+
+  def project
+    @project
+  end
+end

lib/gitlab/reference_extractor.rb

@@ -71,7 +71,7 @@ module Gitlab
       if entry_project.nil?
         false
       else
-        project.nil? || project.id == entry_project.id
+        project.nil? || entry_project.default_issues_tracker?
       end
     end
   end