vi: don't touch file with :x when modified_count == 0

vi: don't touch file with :x when modified_count == 0

Along with it, there are other changes

 - Check for uppercase X is removed as the expression will be always false and
   :X itself is another totally different command in standard vim
 - The status line will show number of written lines instead of lines requested
   by the colon command.  This is also how the standard vim is doing, though
   the difference is that '!' has to be explicitly specified in vim to allow
   partial writes

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>