It seems that now (in 2019) installing latest GIT client could solve the problem according to comments below. This should be the best solution for now.
I have the same problem as you. This is a bug in git: http://git.661346.n2.nabble.com/BUG-git-submodule-update-is-not-fail-safe-td7574168.html
In short, for your problem, try:
# rm -rf external_libraries/BEACHhtml
# git submodule update
It seems there is something wrong with the previous checkout folder, remove it, and update again solves the problem.