Add .dir-locals.el support for file-less buffers.

Add .dir-locals.el support for file-less buffers.
* files.el (hack-local-variables): Split out code to apply local
variable settings ...
(hack-local-variables-apply): ... here.  New function.
(hack-dir-local-variables): Use the default directory for when the
buffer does not have an associated file.
(hack-dir-local-variables-non-file-buffer): New function.
* diff-mode.el (diff-mode):
* vc-annotate.el (vc-annotate-mode):
* vc-dir.el (vc-dir-mode):
* log-edit.el (log-edit-mode):
* log-view.el (log-view-mode): Call hack-dir-local-variables-non-file-buffer.