As the manual page doesn't say, but the in-code kernel documentation
shows, hardlinking for normal users can fail for a variety of
reasons (including very common situations such as non regular file
or non writable file), if the owner of the file does not match
the user linking (e.g. when checking out a shadow repo with a root-
owned master).
If that happens, fail back silently to copying instead of aborting
the whole operation.