Need vc-make-backup-files-when-linked

[Neal Becker]

It is commonly recommended to use cp -al to make local clones when using 
mercurial.  emacs will not break the hard  links when files are edited, 
because vc-make-backup-files is usually nil.

We should have vc-make-backup-files-when-linked, which defaults to t, so that 
emacs will normally break the links.  This is the behavior users will 
expect.