* lib/backupfile.c (backupfile_internal): Fix bug when DIR_FD
does not specify the working directory, and when RENAME.
Without the bug fix, FILE is treated as relative to the working
directory, not relative to DIR_FD, when renaming FILE.
This bug was introduced when DIR_FD and RENAME were introduced,
in 2018-10-24T02:10:21Z!eggert@cs.ucla.edu.
While we’re at it, when SDIR is nonnegative improve performance a
bit by passing an SDIR-relative old name to renameatu.