Added function ls_files in Gitlab::Git::Repository
Using native git command(git ls-tree) ls_files function ls to return only each file's full paths of a repository
This function is called by Repository within GitLab as @DouweM 's suggestion.
(ref : https://github.com/gitlabhq/gitlabhq/pull/9889 & https://github.com/gitlabhq/gitlabhq#9855)