Use message body for merge request description

When a merge request is created from email, use message body
as merge request description. If message body is empty then
merge request description is still created from the source
branch commit (if there is only single commit in the merge
request).

If message body is empty and there are multiple commits in
the source branch, then merge request description is left empty.

Closes #40968

doc/user/project/merge_requests/index.md

@@ -138,7 +138,8 @@ You can create a new merge request by sending an email to a user-specific email
 address. The address can be obtained on the merge requests page by clicking on
 a **Email a new merge request to this project** button.  The subject will be
 used as the source branch name for the new merge request and the target branch
-will be the default branch for the project.
+will be the default branch for the project. The message body (if not empty)
+will be used as the merge request description.
 
 ## Revert changes
 

lib/gitlab/email/handler/create_merge_request_handler.rb

@@ -55,11 +55,13 @@ module Gitlab
         end
 
         def merge_request_params
-          {
+          params = {
             source_project_id: project.id,
             source_branch: mail.subject,
             target_project_id: project.id
           }
+          params[:description] = message if message.present?
+          params
         end
       end
     end

spec/fixtures/emails/valid_new_merge_request.eml

@@ -16,3 +16,5 @@ X-Sieve: CMU Sieve 2.2
 X-Received: by 10.0.0.1 with SMTP id n7mr11234144ipb.85.1371157428600; Thu,
  13 Jun 2013 14:03:48 -0700 (PDT)
 X-Scanned-By: MIMEDefang 2.69 on IPv6:2001:470:1d:165::1
+
+Merge request description

spec/fixtures/emails/valid_new_merge_request_no_description.eml

+Return-Path: <jake@adventuretime.ooo>
+Received: from iceking.adventuretime.ooo ([unix socket]) by iceking (Cyrus v2.2.13-Debian-2.2.13-19+squeeze3) with LMTPA; Thu, 13 Jun 2013 17:03:50 -0400
+Received: from mail-ie0-x234.google.com (mail-ie0-x234.google.com [IPv6:2607:f8b0:4001:c03::234]) by iceking.adventuretime.ooo (8.14.3/8.14.3/Debian-9.4) with ESMTP id r5DL3nFJ016967 (version=TLSv1/SSLv3 cipher=RC4-SHA bits=128 verify=NOT) for <incoming+gitlabhq/gitlabhq@appmail.adventuretime.ooo>; Thu, 13 Jun 2013 17:03:50 -0400
+Received: by mail-ie0-f180.google.com with SMTP id f4so21977375iea.25 for <incoming+gitlabhq/gitlabhq@appmail.adventuretime.ooo>; Thu, 13 Jun 2013 14:03:48 -0700
+Received: by 10.0.0.1 with HTTP; Thu, 13 Jun 2013 14:03:48 -0700
+Date: Thu, 13 Jun 2013 17:03:48 -0400
+From: Jake the Dog <jake@adventuretime.ooo>
+To: incoming+gitlabhq/gitlabhq+merge-request+auth_token@appmail.adventuretime.ooo
+Message-ID: <CADkmRc+rNGAGGbV2iE5p918UVy4UyJqVcXRO2=otppgzduJSg@mail.gmail.com>
+Subject: feature
+Mime-Version: 1.0
+Content-Type: text/plain;
+ charset=ISO-8859-1
+Content-Transfer-Encoding: 7bit
+X-Sieve: CMU Sieve 2.2
+X-Received: by 10.0.0.1 with SMTP id n7mr11234144ipb.85.1371157428600; Thu,
+ 13 Jun 2013 14:03:48 -0700 (PDT)
+X-Scanned-By: MIMEDefang 2.69 on IPv6:2001:470:1d:165::1

spec/lib/gitlab/email/handler/create_merge_request_handler_spec.rb

@@ -49,6 +49,7 @@ describe Gitlab::Email::Handler::CreateMergeRequestHandler do
         expect(merge_request.author).to eq(user)
         expect(merge_request.source_branch).to eq('feature')
         expect(merge_request.title).to eq('Feature added')
+        expect(merge_request.description).to eq('Merge request description')
         expect(merge_request.target_branch).to eq(project.default_branch)
       end
     end
@@ -79,6 +80,17 @@ describe Gitlab::Email::Handler::CreateMergeRequestHandler do
           expect { receiver.execute }.to raise_error(Gitlab::Email::InvalidMergeRequestError)
         end
       end
+
+      context "when the message body is blank" do
+        let(:email_raw) { fixture_file("emails/valid_new_merge_request_no_description.eml") }
+
+        it "creates a new merge request with description set from the last commit" do
+          expect { receiver.execute }.to change { project.merge_requests.count }.by(1)
+          merge_request = project.merge_requests.last
+
+          expect(merge_request.description).to eq('Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>')
+        end
+      end
     end
   end
 end