Added a FUNCS variable to hook environment for consistent sourcing.

Previously the installation path of giddyup was duplicated in each hook
at the sourcing of functions.sh, e.g.

    . /path/to/install/functions.sh

In cases where one hook was deployed to multiple environements, each
installed in different locations, this made sourcing functions.sh
impossible. Now update-hook provides giddyup hooks with FUNCS which
points to functions.sh so sourcing becomes

    . $FUNCS

although the former method is not affected.