pgsql: Fix three blips with progress reporting data (heap and GIN)

Fix three blips with progress reporting data (heap and GIN)

This commit addresses three gaps in the error reporting data, as of:
- In _gin_parallel_merge(), for a parallel GIN build, the last key was
counted twice in the progress report. Oversight in 8492feb98f6.
- _gin_process_worker_data() forgot to increment bs_numtuples when
trimming a list of pointers. Oversight in b229c1016477.
- blocks_done for an index build's heap scan was incorrect for its first
block. There was a one-off in heapam_scan_get_blocks_done(), causing
the routine to return the total number of blocks on the first block
scanned. Oversight in ab0dfc961b6.

Manu has reported two issues. I have come across the one in
_gin_process_worker_data() while reviewing his patches.

All of these are mostly cosmetic, leading to incorrect data reported in
the progress views for a short period of time, so no backpatch is done.

Author: Manu <manuelreyesbravo(at)gmail(dot)com>
Author: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/179012373464.1732093.11171496412950256250@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3be29d9346db1d3225773e2a8e5a86b875696617

Modified Files
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src/backend/access/gin/gininsert.c | 5 +----
src/backend/access/heap/heapam_handler.c | 2 +-
2 files changed, 2 insertions(+), 5 deletions(-)