gc.sh: do not lose a detached HEAD

gc.sh: do not lose a detached HEAD

Officially Girocco does not support any repositories having anything
other than a symbolic-ref HEAD into the refs/heads/... namespace.

However, there's nothing to stop a perverse adminstrator from
creating a detached HEAD using the command line.

Make sure that any such detached HEAD ends up in the main (and
bitmapped for non-forks) pack during a repack operation.

Without this change if a detached HEAD were otherwise unreachable
it would eventually be lost.

But henceforth it shall be lost no more.

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