Improve ibuffer-diff-with-file

Improve ibuffer-diff-with-file

* lisp/ibuf-ext.el (ibuffer-diff-with-file): Link to diff-command in
docstring.  Make Diff buffer read-only from outset and inhibit as
needed to avoid surprises.  Check whether diff-command
supports --label.  Leave point at BOB and clean up any excess
newline inserted by ibuffer-diff-buffer-with-file-1.  Prefer
pop-to-buffer-same-window over switch-to-buffer.

(ibuffer-diff-buffer-with-file-1): Add docstring.  Remove unused
unwind-protect and copypasta from diff-no-select (bug#62599).  Use
diff-file-local-copy, string-join, and redisplay in place of
analogues.  Condition --label use on availability, and label buffers
consistently with diff-no-select.  Leave empty line between runs.
Let diff-sentinel delete temporary files.  Leave point at EOB for
next run.