shlib.sh: always run with `gc.autodetach=false`
Girocco manages all garbage collection operations and they are
always synchronous.
However, if the unthinkable somehow happened and Girocco somehow
actually induced Git into trying to perform garbage collection,
bad things could happen if it ended up proceeding in the background.
Much less likely to fail horribly if it remains in the foreground.
Therefore always add `gc.autodetach=false` to the active config.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>