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
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/bf3b85c6bed2e6b2c104c21d18e78bdb3bcdfb06

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(-)