describe: Refresh the index when run with --dirty

describe: Refresh the index when run with --dirty

When running git describe --dirty the index should be refreshed.  Previously
the cached index would cause describe to think that the index was dirty when,
in reality, it was just stale.

The issue was exposed by python setuptools which hardlinks files into another
directory when building a distribution.

Signed-off-by: Junio C Hamano <gitster@pobox.com>