pgsql: Fix cascading privilege revoke to notice when privileges are sti

Fix cascading privilege revoke to notice when privileges are still held.

If we revoke a grant option from some role X, but X still holds the option
via another grant, we should not recursively revoke the privilege from
role(s) Y that X had granted it to. This was supposedly fixed as one
aspect of commit 4b2dafcc0b1a579ef5daaa2728223006d1ff98e9, but I must not
have tested it, because in fact that code never worked: it forgot to shift
the grant-option bits back over when masking the bits being revoked.

Per bug #6728 from Daniel German. Back-patch to all active branches,
since this has been wrong since 8.0.

Branch
------
REL8_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/d6a3863506599a4d22dd62e0601da9ab2ed933da

Modified Files
--------------
src/backend/utils/adt/acl.c | 4 +-
src/test/regress/expected/privileges.out | 50 ++++++++++++++++++++++++++++++
src/test/regress/sql/privileges.sql | 24 ++++++++++++++
3 files changed, 76 insertions(+), 2 deletions(-)