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@ato.waw.pl>