ci/install-depends: attempt to fix "brew cask" stuff
We run "git pull" against "$cask_repo"; clarify that we are
expecting not to have any of our own modifications and running "git
pull" to merely update, by passing "--ff-only" on the command line.
Also, the "brew cask install" command line triggers an error message
that says:
Error: Calling brew cask install is disabled! Use brew install
[--cask] instead.
In addition, "brew install caskroom/cask/perforce" step triggers an
error that says:
Error: caskroom/cask was moved. Tap homebrew/cask instead.
Attempt to see if blindly following the suggestion in these error
messages gets us into a better shape.
Signed-off-by: Junio C Hamano <gitster@pobox.com>