title: Add "git subtree" when the folder already existing with some file
body: <p>I have a git repo "A" in which repo "B" is added as subtree by team member. I have clone the repository. Now there is a update in repo "B" and I want to pull those changes to my repository "A". I am unable to add repo "B" as subtree in my local hence unable to pull the updates. I need help on this.</p>
  
  <p>Thanks in advance.</p>