update.sh: remove commented-out lines
update.sh: exit 0 if leaderboard.js hasn't changed
update.sh: use absolute paths instead of cd'ing

Default paths:

LEADERBOARD: the root of this repository, for finding scripts and
storing the indermediate table (before diffing with old data)

DATAROOT: where the current and older outputs of the table-creating perl
scripts will be held

That is, LEADERBOARD was originally Matt's ~/code/mt-class, and DATAROOT
was originally Matt's ~/public_html/mt-class.
update.sh: replace UUOC and perl with sed

UUOC = "useless use of cat"
update.sh: make sure $DATAROOT exists.
update.sh: just mv if there's no old data

We check to make sure $DATAROOT/leaderboard.js exists. If it doesn't,
just mv the newly-generated table to that spot.