pgsql: Don't reset changes_since_analyze after a selective-columns ANAL

Don't reset changes_since_analyze after a selective-columns ANALYZE.

If we ANALYZE only selected columns of a table, we should not postpone
auto-analyze because of that; other columns may well still need stats
updates. As committed, the counter is left alone if a column list is
given, whether or not it includes all analyzable columns of the table.
Per complaint from Tomasz Ostrowski.

It's been like this a long time, so back-patch to all supported branches.

Report: <ef99c1bd-ff60-5f32-2733-c7b504eb960c(at)ato(dot)waw(dot)pl>

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/a4400c251f93df7d4f5760ad633c041fc5dae376

Modified Files
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src/backend/commands/analyze.c | 7 +++++--
src/backend/postmaster/pgstat.c | 15 +++++++++++----
src/include/pgstat.h | 4 +++-
3 files changed, 19 insertions(+), 7 deletions(-)