pgsql: Avoid duplicates in ALTER ... DEPENDS ON EXTENSION

Avoid duplicates in ALTER ... DEPENDS ON EXTENSION

If the command is attempted for an extension that the object already
depends on, silently do nothing.

In particular, this means that if a database containing multiple such
entries is dumped, the restore will silently do the right thing and
record just the first one.  (At least, in a world where pg_dump does
dump such entries -- which it doesn't currently, but it will.)

Backpatch to 9.6, where this kind of dependency was introduced.

Reviewed-by: Ibrar Ahmed, Tom Lane (offlist)
Discussion: https://postgr.es/m/20200217225333.GA30974@alvherre.pgsql

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/73b31e784d64a4f24644de11d02fa632fe97bfab

Modified Files
--------------
src/backend/catalog/pg_depend.c                    | 43 ++++++++++++++++++++++
src/backend/commands/alter.c                       |  7 +++-
src/include/catalog/dependency.h                   |  2 +
.../test_extensions/expected/test_extdepend.out    |  2 +
.../modules/test_extensions/sql/test_extdepend.sql |  2 +
5 files changed, 55 insertions(+), 1 deletion(-)