New command: revert-buffer-with-fine-grain

New command: revert-buffer-with-fine-grain

* doc/emacs/files.texi (Reverting): Document the new command and the
new variable.

* etc/NEWS: Mention the new command and the new variable.

* lisp/files.el (revert-buffer-with-fine-grain): New command.  Revert
a buffer trying to be non-destructive, by using replace-buffer-contents.
(revert-buffer-insert-file-contents-delicately): New function, alternative
to revert-buffer-insert-file-contents-function--default-function.
(revert-buffer-with-fine-grain-max-seconds): New variable.  Passed as
argument MAX-SECS of replace-buffer-contents.

* test/lisp/files-tests.el (files-tests-lao files-tests-tzu): Helper
variables, taken from diffutils manual, to test reverting a buffer.
(files-tests-revert-buffer)
(files-tests-revert-buffer-with-fine-grain): New tests (bug#18).