Intead of showing 404 give users ability to close MR with missing branches

app/assets/stylesheets/common.scss

@@ -543,3 +543,15 @@ img.emoji {
 .appear-data {
   display: none;
 }
+
+.label-branch {
+  @include border-radius(4px);
+  padding: 2px 4px;
+  border: none;
+  font-size: 14px;
+  background: #474D57;
+  color: #fff;
+  font-family: $monospace_font;
+  text-shadow: 0 1px 1px #111;
+  font-weight: normal;
+}

app/assets/stylesheets/sections/merge_requests.scss

@@ -70,16 +70,6 @@ li.merge_request {
   @extend .append-bottom-10;
 }
 
-.label_branch {
-  @include border-radius(4px);
-  padding: 2px 4px;
-  border: none;
-  font-size: 14px;
-  background: #474D57;
-  color: #fff;
-  font-family: $monospace_font;
-}
-
 .mr_source_commit,
 .mr_target_commit {
   .commit {

app/controllers/merge_requests_controller.rb

@@ -129,11 +129,11 @@ class MergeRequestsController < ProjectResourceController
 
   def validates_merge_request
     # Show git not found page if target branch doesn't exist
-    return git_not_found! unless @project.repo.heads.map(&:name).include?(@merge_request.target_branch)
+    return invalid_mr unless @project.repo.heads.map(&:name).include?(@merge_request.target_branch)
 
     # Show git not found page if source branch doesn't exist
     # and there is no saved commits between source & target branch
-    return git_not_found! if !@project.repo.heads.map(&:name).include?(@merge_request.source_branch) && @merge_request.commits.blank?
+    return invalid_mr if !@project.repo.heads.map(&:name).include?(@merge_request.source_branch) && @merge_request.commits.blank?
   end
 
   def define_show_vars
@@ -158,4 +158,9 @@ class MergeRequestsController < ProjectResourceController
 
     can?(current_user, action, @project)
   end
+
+  def invalid_mr
+    # Render special view for MR with removed source or target branch
+    render 'invalid'
+  end
 end

app/views/compare/_form.html.haml

@@ -2,9 +2,9 @@
   - unless params[:to]
     %p.slead
       Fill input field with commit id like
-      %code.label_branch 4eedf23
+      %code.label-branch 4eedf23
       or branch/tag name like
-      %code.label_branch master
+      %code.label-branch master
       and press compare button for commits list, code diff.
 
     %br

app/views/merge_requests/invalid.html.haml

+.merge-request
+  = render "merge_requests/show/mr_title"
+  = render "merge_requests/show/mr_box"
+
+  .alert.alert-error
+    %h5
+      %i.icon-exclamation-sign
+      We cannot find
+      %span.label-branch= @merge_request.source_branch
+      or
+      %span.label-branch= @merge_request.target_branch
+      branches in the repository.
+    %p
+      Maybe it was removed or never pushed.
+    %p
+      Please close Merge Request or change branches with existing one
+

app/views/merge_requests/show/_mr_title.html.haml

 %h3.page_title
   = "Merge Request ##{@merge_request.id}:"
    
-  %span.label_branch= @merge_request.source_branch
+  %span.label-branch= @merge_request.source_branch
   →
-  %span.label_branch= @merge_request.target_branch
+  %span.label-branch= @merge_request.target_branch
 
   %span.pull-right
     - if can?(current_user, :modify_merge_request, @merge_request)