[git] Trouble setting up git with my GitHub Account error: could not lock config file

In my case, the .git/config file was created not in my $HOME directory, but inside my repository on .git/config.lock

Deleting the file fixed the problem. The file is created, when I switch branches with git checkout -f branchname and files from a recently created submodule were overwritten by git checkout.

$ git branch --set-upstream-to=origin/branchname
error: could not lock config file .git/config: File exists
error: Unable to write upstream branch configuration
hint: 
hint: After fixing the error cause you may try to fix up
hint: the remote tracking information by invoking
hint: "git branch --set-upstream-to=origin/branchname".