Re: fix tablespace handling in pg_combinebackup

I don't know how to fix 82023d47^ on Windows[1][2], but in case it's
useful, here's a small clue.  I see that Perl's readlink() does in
fact know how to read "junction points" on Windows[3], so if that was
the only problem it might have been very close to working.  One
difference is that our own src/port/dirmod.c's pgreadlink() also
strips \??\ from the returned paths (something to do with the
various forms of NT path), but when I tried that:

                        my $olddir = readlink("$backup_path/pg_tblspc/$tsoid")
                                || die "readlink
$backup_path/pg_tblspc/$tsoid: $!";

+                       # strip NT path prefix (see src/port/dirmod.c
pgreadlink())
+                       $olddir =~ s/^\\\?\?\\// if
$PostgreSQL::Test::Utils::windows_os;

... it still broke[4].  So I'm not sure what's going on...

[1] https://github.com/postgres/postgres/runs/24040897199
[2]
https://api.cirrus-ci.com/v1/artifact/task/5550091866472448/testrun/build/testrun/pg_combinebackup/002_compare_backups/log/002_compare_backups_pitr1.log
[3] https://github.com/Perl/perl5/blob/f936cd91ee430786a1bb6068a4a7c8362610dd5f/win32/win32.c#L2041
[4] https://cirrus-ci.com/task/6746621638082560