applying .dir-locals.el settings when buffer-file-name is nil

[Dan Nicolaescu]

.dir-locals.el settings do not apply to buffers when (null
buffer-file-name) even when `default-directory' for those buffers is
dominated by .dir-locals.el.

VC for example creates buffers for which it would be good to be able to
have .dir-locals.el.  Examples: the log-edit-mode buffers - be able to
set some checkin rules, or automatically add things to the commit
message,  the vc-diff buffers  - show trailing whitespace, etc.