Implement :x rigorously

[lloeki]

Shouldn't update mtime when no changes were made, whereas :w always writes (so, touches the file).

For reference:

:[range]x[it][!] [++opt] [file]
            Like ":wq", but write only when changes have been made.

Also, some peculiarities:

$ vim foo bar
:xa
# exits and nothing written
$ vim foo bar
:x
# warns and nothing written
$ vim foo bar
:wqa
# weirdly, exits and nothing written
$ vim foo bar
:wq
# writes foo, then warns