pgsql: pg_combinebackup: Fix incorrect tablespace handling.

pg_combinebackup: Fix incorrect tablespace handling.

The previous coding mangled the pathname calculation for
incremental files located in user-defined tablespaces.

Enhance the test cases to cover such cases, as I should have
done originally. Thanks to Andres Freund for alerting me to the
lack of test coverage.

Discussion: http://postgr.es/m/CA+TgmoYdXTjo9iQeoipTccDpWZzvBNS6EndY2uARM+T4yG_yDg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cd64dc42d1e1b03e57e6ba3d316e4f9dec52a78d

Modified Files
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src/bin/pg_combinebackup/pg_combinebackup.c       |  2 +-
src/bin/pg_combinebackup/reconstruct.c            |  7 ++---
src/bin/pg_combinebackup/t/002_compare_backups.pl | 32 ++++++++++++++++++++---
3 files changed, 33 insertions(+), 8 deletions(-)