pgsql: Prevent (auto)vacuum from truncating first page of populated mat

Prevent (auto)vacuum from truncating first page of populated matview.

Per report from Fujii Masao, with regression test using his example.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/b69ec7cc990fd8da75ed4c232899503217d7b9ae

Modified Files
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src/backend/commands/vacuumlazy.c     |    6 ++++++
src/test/regress/expected/matview.out |   20 ++++++++++++++++++++
src/test/regress/sql/matview.sql      |   12 ++++++++++++
3 files changed, 38 insertions(+), 0 deletions(-)