Allow more mail clients to group emails by thread

* send a ‘In-Reply-To’ header along the ‘References’ header
* subject of answers to an existing thread begins with ‘Re: ’

This fixes threading with at least Mail.app and Airmail.

1 files changed, 12 insertions, 11 deletions

diff --git a/app/mailers/emails/notes.rb b/app/mailers/emails/notes.rb
index d6ab63fe68c..7848d34ab2b 100644
--- a/app/mailers/emails/notes.rb
+++ b/app/mailers/emails/notes.rb
@@ -5,9 +5,10 @@ module Emails
       @commit = @note.noteable
       @project = @note.project
       @target_url = project_commit_url(@project, @commit, anchor: "note_#{@note.id}")
-      mail(from: sender(@note.author_id),
-           cc: recipient(recipient_id),
-           subject: subject("#{@commit.title} (#{@commit.short_id})"))
+      mail_answer_thread(@commit,
+                         from: sender(@note.author_id),
+                         cc: recipient(recipient_id),
+                         subject: subject("#{@commit.title} (#{@commit.short_id})"))
     end
 
     def note_issue_email(recipient_id, note_id)
@@ -15,10 +16,10 @@ module Emails
       @issue = @note.noteable
       @project = @note.project
       @target_url = project_issue_url(@project, @issue, anchor: "note_#{@note.id}")
-      set_reference("issue_#{@issue.id}")
-      mail(from: sender(@note.author_id),
-           cc: recipient(recipient_id),
-           subject: subject("#{@issue.title} (##{@issue.iid})"))
+      mail_answer_thread(@issue,
+                         from: sender(@note.author_id),
+                         cc: recipient(recipient_id),
+                         subject: subject("#{@issue.title} (##{@issue.iid})"))
     end
 
     def note_merge_request_email(recipient_id, note_id)
@@ -26,10 +27,10 @@ module Emails
       @merge_request = @note.noteable
       @project = @note.project
       @target_url = project_merge_request_url(@project, @merge_request, anchor: "note_#{@note.id}")
-      set_reference("merge_request_#{@merge_request.id}")
-      mail(from: sender(@note.author_id),
-           cc: recipient(recipient_id),
-           subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
+      mail_answer_thread(@merge_request,
+                         from: sender(@note.author_id),
+                         cc: recipient(recipient_id),
+                         subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
     end
 
     def note_wall_email(recipient_id, note_id)