cp -up: preserve all hard links

* src/copy.c (copy_internal): With --update (-u), this function would
return early once it found that the destination is not older than the
source, *without* recording the source-dev/ino--to--dest_name mapping.
That mapping is required in order to preserve src hard links in the
destination tree, so when using cp with --update and --preserve=links
(perhaps via -p or -a), cp could fail to preserve one hard link
per inode when at least one of the hard-linked names already exists
in the destination tree.
Reported by Odd Harry Mannsverk in http://debbugs.gnu.org/8419.
* tests/cp/preserve-link: New file.  Exercise the flaw/fix.
* tests/Makefile.am (TESTS): Add it.
* NEWS (Bug fixes): Mention it.

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