Merge branch 'dz-mr-version-compare' into 'master'

Allow compare merge request versions

## What does this MR do?

Add new functionality to the merge request page. It allows you easily compare merge request versions in one click.  

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

To improve code review experience. 

## Screenshots (if relevant)

See discussion

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] ~~API support added~~
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/13570

See merge request !6127

1 files changed, 26 insertions, 5 deletions

diff --git a/app/controllers/projects/merge_requests_controller.rb b/app/controllers/projects/merge_requests_controller.rb
index 4f9ca0097a1..8895cb955bd 100644
--- a/app/controllers/projects/merge_requests_controller.rb
+++ b/app/controllers/projects/merge_requests_controller.rb
@@ -90,16 +90,27 @@ class Projects::MergeRequestsController < Projects::ApplicationController
         @merge_request.merge_request_diff
       end
 
+    @merge_request_diffs = @merge_request.merge_request_diffs.select_without_diff
+    @comparable_diffs = @merge_request_diffs.select { |diff| diff.id < @merge_request_diff.id }
+
+    if params[:start_sha].present?
+      @start_sha = params[:start_sha]
+      @start_version = @comparable_diffs.find { |diff| diff.head_commit_sha == @start_sha }
+
+      unless @start_version
+        render_404
+      end
+    end
+
     respond_to do |format|
       format.html { define_discussion_vars }
       format.json do
-        unless @merge_request_diff.latest?
-          # Disable comments if browsing older version of the diff
-          @diff_notes_disabled = true
+        if @start_sha
+          compared_diff_version
+        else
+          original_diff_version
         end
 
-        @diffs = @merge_request_diff.diffs(diff_options)
-
         render json: { html: view_to_html_string("projects/merge_requests/show/_diffs") }
       end
     end
@@ -529,4 +540,14 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     params[:merge_request] ||= ActionController::Parameters.new(source_project: @project)
     @merge_request = MergeRequests::BuildService.new(project, current_user, merge_request_params).execute
   end
+
+  def compared_diff_version
+    @diff_notes_disabled = true
+    @diffs = @merge_request_diff.compare_with(@start_sha).diffs(diff_options)
+  end
+
+  def original_diff_version
+    @diff_notes_disabled = !@merge_request_diff.latest?
+    @diffs = @merge_request_diff.diffs(diff_options)
+  end
 end