Users who do not have clang-format installed were not able to push commits. This patch gives
better feedback to the developer by explicitely telling the user to install the package.
This allows importing of external libraries into externals/ without
having to reformat them. Unfortunately it also allows whitespace to be
introduced in files like the root CMakeLists.txt, but that is a small
downside compared to the tradeoff.
Remove unneeded stuff from pre-commit script
The check against an empty commit was not needed (it is only a security for the 1st commit after git init).
It could also possibly pose problems because of the redirection to /dev/null on some windows systems.
newline at EOF & fixed indent
