generate-git-snapshot: Support PRE_DCH_HOOK variable for custom scripts

This allows stuff like:

  echo "sed -i 's/^Section: _SECTION_/Section: dev/' debian/control" > /tmp/changeVersion.sh
  export PRE_DCH_HOOK="/tmp/changeVersion.sh"
  /usr/bin/generate-git-snapshot

Thanks: Rudolph Bott <bott@sipgate.de> for the patch