gc.sh: add girocco.redelta escape hatch

gc.sh: add girocco.redelta escape hatch

Normally, generating new deltas when there are only a small number of
objects present is the right thing to do as it's fast and helps to
increase efficiency of the hosted repositories.

However, in the case where there is some unexpected pathological redelta
behavior, this automatic recompute-deltas-for-small-number-of-objects
behavior can be disabled by explicitly setting the config variable
"girocco.redelta" to false.

To disable redeltas for all projects by default, "girocco.redelta" can
be set to false in the $Girocco::Config::chroot/etc/girocco/.gitconfig
file and then explicitly set to true for individual projects where the
redelta behavior is desired.

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