Cleanup diff-buffer-with-file.

Cleanup diff-buffer-with-file.
* lisp/vc/diff.el (diff-old-temp-file, diff-new-temp-file): Remove.
(diff-sentinel): Get them as arguments instead.
(diff-old-file, diff-new-file, diff-extra-args): Remove.
(diff-file-local-copy, diff-better-file-name): New funs.
(diff-no-select): Rename from diff-into-buffer.
Support buffers additionally to files.  Move `buf' arg.  Don't display buf.
Prefer closures to buffer-local variables.
(diff): Adjust accordingly.
(diff-buffer-with-file): Move from files.el.
* lisp/files.el (diff-buffer-with-file): Move to vc/diff.el.
(diff-buffer-internal): Remove.
(diff-buffer-buffer): Remove.
(save-some-buffers-action-alist): Use diff-no-select so as not to guess
the buffer name used, and so as not to mess up windows and frames.