update.sh: move quarantine creation into function

update.sh: move quarantine creation into function

Move creation of the quarantine area into its own function.

This makes it possible to selectively use the quarantine feature
depending on the type of the source repository being fetched.

This is necessary because foreign vcs fetches may not easily
support fetching into a quarantine area.  While it's certainly
possible to fetch the git portion of a foreign vcs fetch into
a quarantine area, if the fetch fails the quarantine and the
quarantined objects are discarded, it can be very complicated
to reset the foreign vcs part of the fetch to avoid corruption.

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