[OpenSmalltalk/opensmalltalk-vm] 07aabb: Let the .git/hooks work for git 2.18 and later

 
  Branch: refs/heads/Cog
  Home:   https://github.com/OpenSmalltalk/opensmalltalk-vm
  Commit: 07aabbaf15c6eac285faea94d370a7a373da69ab
      https://github.com/OpenSmalltalk/opensmalltalk-vm/commit/07aabbaf15c6eac285faea94d370a7a373da69ab
  Author: Nicolas Cellier <[hidden email]>
  Date:   2019-04-11 (Thu, 11 Apr 2019)

  Changed paths:
    M scripts/updateSCCSVersions

  Log Message:
  -----------
  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