hook: fix GIT_DIR in delegate script

$PWD gives us the absolute path to the git repository without using the
bash builtin.

Signed-off-by: oppiliappan <[e-mail]>

hook/setup.go

···
 data=$(cat)
 exitcodes=""
 hookname=$(basename $0)
-GIT_DIR=${GIT_DIR:-$(dirname $0)/..}
+GIT_DIR="$PWD"
 
 for hook in ${GIT_DIR}/hooks/${hookname}.d/*; do
   test -x "${hook}" && test -f "${hook}" || continue