Variable: break-hardlink-on-save

Non-nil means when saving a file that exists under several names
(i.e., has multiple hardlinks), break the hardlink associated with
`buffer-file-name' and write to a new file, so that the other
instances of the file are not affected by the save.

If `buffer-file-name' refers to a symlink, do not break the symlink.

Unlike `file-precious-flag', `break-hardlink-on-save' is not advisory.
For example, if the directory in which a file is being saved is not
itself writable, then error instead of saving in some
hardlink-nonbreaking way.

See also `backup-by-copying' and `backup-by-copying-when-linked'.