gc.sh: add repack dir links to logs and worktrees

gc.sh: add repack dir links to logs and worktrees

When running pack-objects from the repack subdirectory created for
use during repacking, the "--reflog" option will not be effective
unless it can actually see the logs.

Add a link back to the logs directory if one exists so this can
happen and the referenced objects can be kept.

Unfortunately linked working tree HEAD logs seem to be left out in
the cold by the "--reflog" option.  Nevertheless, go ahead and add
a link back to the worktrees directory if one exists in case a
future Git version remedies the problem.

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