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This changes the behavior of rename-file etc. slightly.
The old behavior mostly disagreed with the documentation, and had
a race condition bug that could allow attackers to modify victims'
write-protected directories (Bug#27986).
* doc/lispref/files.texi (Changing Files): Document that in
rename-file etc., NEWFILE is special if it is a directory name.
* etc/NEWS: Document the change in behavior.
* src/fileio.c (directory_like): Remove.  All uses removed.
(expand_cp_target): Test only whether NEWNAME is a directory name,
not whether it is currently a directory.  This avoids a race.
(Fcopy_file, Frename_file, Fadd_name_to_file, Fmake_symbolic_link):
Document behavior if NEWNAME is a directory name.
(Frename_file): Simplify now that the destdir behavior occurs
only when NEWNAME is a directory name.
* test/lisp/net/tramp-tests.el (tramp-test11-copy-file)
(tramp-test12-rename-file, tramp--test-check-files):
Adjust tests to match new behavior.