update.sh: replace "show_progress=1 bang ..." with "bang_eval ..."

update.sh: replace "show_progress=1 bang ..." with "bang_eval ..."

When computing the before and after refs on a mirror update in order
to trigger the appropriate ref-change actions, it's necessary to
redirect the output of some commands.  The only way to guarantee
that output doesn't get swallowed into the bang log when running the
command using bang is to set the show_progress variable.  This has
the unfortunate effect of also sending all that output into the bang
log as well.

Replace the "show_progress=1 bang CMD..." commands that use redirection
with equivalent "bang_eval 'CMD...'" commands so that a well-defined
failure mode can be maintained, the redirected output goes where it
needs to and unnecessarily output does not end up in the bang log.