Re: pg_basebackup check vs Windows file path limits

Since this test is passing on HEAD which has slightly shorter paths, I'm wondering if we should change this:

rename("$pgdata/pg_replslot", "$tempdir/pg_replslot")
  or BAIL_OUT "could not move $pgdata/pg_replslot";
dir_symlink("$tempdir/pg_replslot", "$pgdata/pg_replslot")
  or BAIL_OUT "could not symlink to $pgdata/pg_replslot";

to use the much shorter $sys_tempdir created a few lines below.

Pushed a tested fix along those lines.

Today I've started up my Windows VM to run some tests and discovered a test
failure caused by that fix (e213de8e7):
>meson test
Ok:                 246
Expected Fail:      0
Fail:               1
Unexpected Pass:    0
Skipped:            14
Timeout:            0

...\010_pg_basebackup\log\regress_log_010_pg_basebackup.txt contains:
[04:42:45.321](0.291s) Bail out!  could not move T:\postgresql\build/testrun/pg_basebackup/010_pg_basebackup\data/t_010_pg_basebackup_main_data/pgdata/pg_replslot

With a diagnostic print added before rename() in 010_pg_basebackup.pl, I see:
rename("T:\postgresql\build/testrun/pg_basebackup/010_pg_basebackup\data/t_010_pg_basebackup_main_data/pgdata/pg_replslot", "C:\Users\User\AppData\Local\Temp\fGT76tZUWr/pg_replslot")
That is, I have the postgres source tree and the user tempdir placed on
different disks.

perldoc on rename() says that it usually doesn't work across filesystem
boundaries, so I think it's not a Windows-specific issue.

Best regards,
Alexander