Prevent "cannot lock ref" git error

```
> git pull origin master
From github.com:Microsoft/vscode
 * branch                  master     -> FETCH_HEAD
error: cannot lock ref 'refs/remotes/origin/master': is at 2e4bfdb24fd137a1d2e87bd480f283cf7001f19a but expected 70ea06a46fd4b38bdba9ab1d64f3fee0f63806a5
 ! 70ea06a46f..2e4bfdb24f  master     -> origin/master  (unable to update local ref)
```