Refactor tar method of walmethods.c to rely on the compression method

Since d62bcc8, the directory method of walmethods.c uses the compression
method to determine which code path to take.  The tar method, used by
pg_basebackup --format=t, was inconsistent regarding that, as it relied
on the compression level to check if no compression or gzip should be
used.  This commit makes the code more consistent as a whole in this
file, making the tar logic use a compression method rather than
assigning COMPRESSION_NONE that would be ignored.

The options of pg_basebackup are planned to be reworked but we are not
sure yet of the shape they should have as this has some dependency with
the integration of the server-side compression for base backups, so this
is left out for the moment.  This change has as benefit to make easier
the future integration of new compression methods for the tar method of
walmethods.c, for the client-side compression.

Reviewed-by: Georgios Kokolatos
Discussion: https://postgr.es/m/Yb3GEgWwcu4wZDuA@paquier.xyz

src/bin/pg_basebackup/pg_basebackup.c

@@ -524,7 +524,8 @@ LogStreamerMain(logstreamer_param *param)
 													stream.do_sync);
 	else
 		stream.walmethod = CreateWalTarMethod(param->xlog,
-											  COMPRESSION_NONE, /* ignored */
+											  (compresslevel > 0) ?
+											  COMPRESSION_GZIP : COMPRESSION_NONE,
 											  compresslevel,
 											  stream.do_sync);