gc.sh: support girocco.redelta=always config
If `git config girocco.redelta` is "always" then for the final repack on
a full gc, deltas will always be recomputed regardless of the value of
$var_redelta_threshold or how many objects are actually present.
This can be used to force a repository to stay as optimized as possible
even if it execeeds the normal redelta threshold.
However, care must be used as setting girocco.redelta=always on a
repository with an extremely excessive number of objects will likely
cause either a memory blowout or a gc timeout.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>