Correctly set comparison first commit when range includes a merge commit

app/controllers/projects/compare_controller.rb

@@ -19,8 +19,8 @@ class Projects::CompareController < Projects::ApplicationController
     if compare_result
       @commits = Commit.decorate(compare_result.commits, @project)
       @diffs = compare_result.diffs
-      @commit = @commits.last
-      @first_commit = @commits.first
+      @commit = @project.commit(head_ref)
+      @first_commit = @project.commit(base_ref)
       @line_notes = []
     end
   end

app/helpers/diff_helper.rb

@@ -171,7 +171,7 @@ module DiffHelper
   def commit_for_diff(diff)
     if diff.deleted_file
       first_commit = @first_commit || @commit
-      first_commit.parent
+      first_commit.parent || @first_commit
     else
       @commit
     end