Link to conflicting MR in reopening error message

Also, the error message became shorter.

app/controllers/projects/merge_requests_controller.rb

@@ -7,6 +7,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
   include RendersCommits
   include ToggleAwardEmoji
   include IssuableCollections
+  include MarkupHelper
 
   skip_before_action :merge_request, only: [:index, :bulk_update]
   before_action :whitelist_query_limiting, only: [:assign_related_issues, :update]
@@ -122,9 +123,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
 
     respond_to do |format|
       format.html do
-        if merge_request_params[:state_event] && @merge_request.errors.any?
-          flash[:alert] = @merge_request.errors.values.flatten.to_sentence
-        end
+        check_branch_conflict
 
         if @merge_request.valid?
           redirect_to([@merge_request.target_project.namespace.becomes(Namespace), @merge_request.target_project, @merge_request])
@@ -259,6 +258,12 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
     @merge_request.check_if_can_be_merged
   end
 
+  def check_branch_conflict
+    if @merge_request.errors[:validate_branches]
+      flash[:alert] = markdown(@merge_request.errors[:validate_branches].to_sentence, pipeline: :single_line)
+    end
+  end
+
   def merge!
     # Disable the CI check if merge_when_pipeline_succeeds is enabled since we have
     # to wait until CI completes to know

app/models/merge_request.rb

@@ -547,7 +547,7 @@ class MergeRequest < ActiveRecord::Base
       similar_mrs = similar_mrs.where('id not in (?)', self.id) if self.id
       if similar_mrs.any?
         errors.add :validate_branches,
-                   "Cannot open this Merge Request because another open Merge Request already exists for this source branch: #{similar_mrs.first.title}"
+                   "Another open Merge Request already exists for this source branch: !#{similar_mrs.first.id}"
       end
     end
   end

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -298,7 +298,7 @@ describe Projects::MergeRequestsController do
 
         update_merge_request(state_event: 'reopen')
 
-        expect(controller).to set_flash[:alert].to(/another open Merge Request already exists for this source branch/)
+        expect(controller).to set_flash[:alert].to(/Another open Merge Request already exists for this source branch/)
         expect(merge_request.reload).to be_closed
       end
     end