Make levels 1-based in pg_log_backend_memory_contexts()

Make levels 1-based in pg_log_backend_memory_contexts()

Both pg_get_process_memory_contexts() and pg_backend_memory_contexts
have 1-based levels, whereas pg_log_backend_memory_contexts() was using
0-based levels.  Align these.

This results in slightly saner behavior from MemoryContextStatsDetail()
in regards to the max_level.  Previously it would stop at 1 level before
the maximum requested level rather than at that level.

Reported-by: Atsushi Torikoshi <[email protected]>
Author: Atsushi Torikoshi <[email protected]>
Author: David Rowley <[email protected]
Reviewed-by: Melih Mutlu <[email protected]>
Reviewed-by: Rahila Syed <[email protected]>
Discussion: https://postgr.es/m/395ea5d4fe190480efa95bf533485c70@oss.nttdata.com