private/graph: Fix a couple of regression

Since the perlcritic fixes, the --longest-path option has failed to
highlight the longest path.  This was caused by "$#x..0" producing an
empty sequence (rather than a "from $#x to 0 by -1").

Secondly, with the introduction of helper tools, the script would fail
due to helper tools not being locatable.

Signed-off-by: Niels Thykier <niels@thykier.net>