pg_combinebackup PITR comparison test fix

Hi hackers,

While I was going through the TAP tests to fix the formatting of command
argument lists to be less confusing, I noticed that pg_combinebackup's
002_compare_backups.pl test accidentally ran pg_dumpall twice against
the same database, thus rendering the tests useless.

Fixing that revealed that there is a difference in the dumps: the
tablespaces have different paths.  pg_dumpall doesn't have an option to
map tablespace paths, so instead I used File::Compare::compare_text()
with a custom comparison function to erase the difference.

- ilmari