fix update-hook-example to work with packed tag references

The update-hook-example used 'test -f' to check the tag present, which
does not work if the checked reference is packed. This check has been
changed to use 'git rev-parse $tag' instead.

Signed-off-by: Dmitry Potapov <dpotapov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/howto/update-hook-example.txt

@@ -68,7 +68,7 @@ function info {
 # - Branches should only be fast-forwarded.
 case "$1" in
   refs/tags/*)
-    [ -f "$GIT_DIR/$1" ] &&
+    git rev-parse --verify -q "$1" &&
     deny >/dev/null "You can't overwrite an existing tag"
     ;;
   refs/heads/*)