Let the .git/hooks work for git 2.18 and later
Let the .git/hooks work for git 2.18 and later

GIT_DIR is not set any longer since git 2.18, so the hook is not recognized as running as hook
Replace with a test of $dirname $0 instead as suggested on SO
https://stackoverflow.com/questions/55627720/how-to-recognize-if-git-hook-script-is-really-run-as-a-hook?noredirect=1#comment97952519_55627720

This should fix issue #389

timfel/squeakvm,OpenSmalltalk/vm,OpenSmalltalk/vm,OpenSmalltalk/vm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,OpenSmalltalk/vm,OpenSmalltalk/vm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,OpenSmalltalk/vm,timfel/squeakvm,OpenSmalltalk/vm,OpenSmalltalk/vm