Restore MERGE_HEAD and CHERRY_PICK_HEAD without newline

Including the trailing newline results in an error on Windows:
  fatal: Corrupt MERGE_HEAD file
  (<commit-hash>)

Author: jawshooah

lib/overcommit/git_repo.rb

@@ -167,7 +167,7 @@ def restore_merge_state
         FileUtils.touch(File.expand_path('MERGE_MODE', Overcommit::Utils.git_dir))
 
         File.open(File.expand_path('MERGE_HEAD', Overcommit::Utils.git_dir), 'w') do |f|
-          f.write("#{@merge_head}\n")
+          f.write(@merge_head)
         end
         @merge_head = nil
       end
@@ -186,7 +186,7 @@ def restore_cherry_pick_state
       if @cherry_head
         File.open(File.expand_path('CHERRY_PICK_HEAD',
                                    Overcommit::Utils.git_dir), 'w') do |f|
-          f.write("#{@cherry_head}\n")
+          f.write(@cherry_head)
         end
         @cherry_head = nil
       end