pg_rewind: Fix thinko in parsing target WAL.
It's entirely possible to see WAL for a relation that doesn't exist in
the target anymore. That happens when the relation was dropped later.
The refactoring in commit
eb00f1d4b broke that case, by sanity-checking
the file type in the target before checking the flag forwhether it
exists there at all.
I noticed this during manual testing. Modify the 001_basic.pl test so
that it covers this case.