update.sh: use "update.pid" lock against simultaneous updates

update.sh: use "update.pid" lock against simultaneous updates

Use the "update.pid" lock file similarly to the way the "gc.pid"
file is used to lock out simultaneous updates of the same project.

The `update.sh` script does not handle having multiple copies
run simultaneously for the same project.

With this change it, all copies but the first will error out.

This, however, does not yet mean that `update.sh` can run
simultaneously on the same project as `gc.sh`.

While theoretically such is possible (since remote pushes into
the repository are always allowed while `gc.sh` is running on
the repository), the `update.sh` script will need a full audit
before allowing such a thing.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>