title: Unable to add git subtree
body: <p>I am trying to establish a git subtree in my project and for some reason I keep getting a message saying <code>You must provide the --prefix option.</code> I am making a Flutter project and the project repo (as well as sub repo) are both on gitlab.</p>
  <p>Below is the command that I am running in the console while in my root project directory.</p>
  <pre><code>git subtree add —-prefix=subtrees/mysubtree https://git.company.com/subdirectory/subtree_repo.git master — squash
  </code></pre>
  <p>I have experimented a little with the command. For instance I tried <code>—-prefix subtrees/mysubtree</code> (sans <code>=</code>), as well as <code>git config alias.sba 'subtree add --prefix subtrees/mysubtree / https://git.company.com/subdirectory/subtree_repo.git master —-squash'</code>.</p>
  <p>The last command doesn't throw any errors and it actually triggers <code>pub get</code> but no files change and no directories are added (at least not to the root which is where I am running the command).</p>
  <p>I am not really sure what I am doing wrong and the messages (when there are messages) aren't very informative. I have checked my syntax and referenced it with <a href="https://docs.gitlab.com/ee/topics/git/subtree.html" rel="nofollow noreferrer">Gitlab</a> as well as with <a href="https://www.netways.de/en/blog/2016/01/14/working-with-git-subtree/#:%7E:text=Purge%20a%20subtree,unlike%20known%20from%20git%20submodules." rel="nofollow noreferrer">this</a> post.</p>