ci(osx): update homebrew-cask repository with less noise

ci(osx): update homebrew-cask repository with less noise

The OSX CI build procedure updates the homebrew-cask repository
before attempting to install perforce again, after seeing an
installation failure.  This involves a "git pull" that by default
computes and outputs diffstat, which would only grow as the time
goes by and the repository cast in stone in the CI build image
becomes more and more stale relative to the upstream repository in
the outside world.

Suppress the diffstat to both save cycles to generate it, and strain
on the eyeballs to skip it.

Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>