* vc/diff.el (diff-better-file-name): Function deleted.
abbreviating file names causes problems with shell-quote-argument.
(diff-no-select): Just use expand-file-name.
* lisp/ibuf-ext.el (diff-sentinel): Update declaration.
* lisp/ibuffer.el: Regenerate autoloads.
* lisp/vc/diff.el (diff-sentinel): Make new arguments optional, eg for
the sake of ibuffer-diff-buffer-with-file-1.
* 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.
(diff-buffer-internal): New function extracted from diff-buffer-with-file
(diff-buffer-with-file): Use it.
* lisp/vc/diff.el (diff-into-buffer): New fun, extracted from diff.
(diff): Use it.
Fixes: debbugs:7277
