Remove @commit in compare and MR controllers

app/controllers/projects/compare_controller.rb

@@ -51,12 +51,9 @@ class Projects::CompareController < Projects::ApplicationController
     if @compare
       @commits = @compare.commits
-      @start_commit = @compare.start_commit
-      @commit = @compare.commit
-
       @diffs = @compare.diffs(diff_options)
-      environment_params = @repository.branch_exists?(@head_ref) ? { ref: @head_ref } : { commit: @commit }
+      environment_params = @repository.branch_exists?(@head_ref) ? { ref: @head_ref } : { commit: @compare.commit }
       @environment = EnvironmentsFinder.new(@project, current_user, environment_params).execute.last
       @diff_notes_disabled = true

app/controllers/projects/merge_requests_controller.rb

@@ -14,7 +14,6 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   ]
   before_action :validates_merge_request, only: [:show, :diffs, :commits, :pipelines]
   before_action :define_show_vars, only: [:diffs, :commits, :conflicts, :conflict_for_path, :builds, :pipelines]
-  before_action :define_commit_vars, only: [:diffs]
   before_action :ensure_ref_fetched, only: [:show, :diffs, :commits, :builds, :conflicts, :conflict_for_path, :pipelines]
   before_action :close_merge_request_without_source_project, only: [:show, :diffs, :commits, :builds, :pipelines]
   before_action :check_if_can_be_merged, only: :show
@@ -130,8 +129,6 @@ class Projects::MergeRequestsController < Projects::ApplicationController
       @diff_notes_disabled = true
     end
-    define_commit_vars
-
     render_diff_for_path(@diffs)
   end
@@ -500,10 +497,6 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     @notes = prepare_notes_for_rendering(@discussions.flat_map(&:notes))
   end
-  def define_commit_vars
-    @commit = @merge_request.diff_head_commit
-  end
-
   def define_diff_vars
     @merge_request_diff =
       if params[:diff_id]

app/models/merge_request_diff.rb

@@ -159,6 +159,9 @@ class MergeRequestDiff < ActiveRecord::Base
   # It is not good enough for highlighting diffs, so we can't simply pass
   # these as `diff_refs.`
   def fallback_diff_refs
+    real_refs = diff_refs
+    return real_refs if real_refs
+
     likely_base_commit_sha = (first_commit&.parent || first_commit)&.sha
     Gitlab::Diff::DiffRefs.new(