Rollback diff drop and set default commit id to HEAD

app/models/repository.rb

@@ -25,7 +25,7 @@ class Repository
     raw_repository.empty?
   end
 
-  def commit(id = nil)
+  def commit(id = 'HEAD')
     return nil unless raw_repository
     commit = Gitlab::Git::Commit.find(raw_repository, id)
     commit = Commit.new(commit) if commit

lib/gitlab/diff/parser.rb

@@ -13,7 +13,7 @@ module Gitlab
 
         lines_arr = ::Gitlab::InlineDiff.processing lines
 
-        lines_arr.drop(4).each do |line|
+        lines_arr.each do |line|
           raw_line = line.dup
 
           next if filename?(line)