In order to keep only the core of pg_rman for incremental/differential
backup, this looks necessary and makes the code more simple. Including
server log files in a backup could be subject to discussion as well,
as for example a Postgres base backup does not include them, just
because in this case server instance is not aware of the log files.
This commit simplifies the way backup sizes are saved internally by
reusing the same variable for incremental and full backup, which were
using separated and exclusively used variables, resulted in a couple
of bytes wasted all the time. This was also reflected by a useless
column in the output table of subcommand "show".
