When trying to checkout the GCC branch, it is unable to update the LLVM submodule.
alpaca@alpaca-VirtualBox:~/alpaca-oopsla2017$ git checkout gcc
M ext/alpaca
Branch gcc set up to track remote branch gcc from origin.
Switched to a new branch 'gcc'
alpaca@alpaca-VirtualBox:~/alpaca-oopsla2017$ git submodule update
fatal: reference is not a tree: 6d28aced987df8418331f6337bc8c22719d72c42
Unable to checkout '6d28aced987df8418331f6337bc8c22719d72c42' in submodule path 'ext/alpaca'
When trying to checkout the GCC branch, it is unable to update the LLVM submodule.