Fix thinko coming from 000f3adf

Michael Paquier <michael@paquier.xyz>

Commit: d0d62262d34154965511cfda6b98609d27752d5a
Author: Michael Paquier <michael@paquier.xyz>
Date: 2022-01-08T00:12:21Z
Releases: 15.0
Fix thinko coming from 000f3adf

pg_basebackup.c relies on the compression level to not be 0 to decide if
compression should be used, but 000f3adf missed the fact that the
default compression (Z_DEFAULT_COMPRESSION) is -1, which would be used
if specifying --gzip without --compress.

While on it, add some coverage for --gzip, as this is rather easy to
miss.

Reported-by: Christoph Berg
Discussion: https://postgr.es/m/YdhRDMLjabtXOnhY@msg.df7cb.de

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