pgsql: Add PageGetPruneXid() helper

Add PageGetPruneXid() helper

This is similar to the other page accessors in bufpage.h. It improves
readability and avoids long lines.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/BD8B69E7-26D8-4706-9164-597C6AE57812%40gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/68c2dcb9130e8e3bc60676909676be1567c3cd51

Modified Files
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src/backend/access/heap/pruneheap.c | 4 ++--
src/include/storage/bufpage.h       | 6 ++++++
2 files changed, 8 insertions(+), 2 deletions(-)