bug#7747: Please put current file name in the prompt for rename-file

[Drew Adams]

> The prompt for `rename-file' does not currently show the old file name

What old file name?  Which file is the "old file"?

> Wouldn't it be better if the prompt should the current file name at init?

What "current file"?

You can use `rename-file' from anywhere in Emacs.  Yes, if you use it from a
file buffer or from Dired then a reasonable default value is provided.  But
that's all it is: a default value.

We should treat this default value the same way we treat default values
generally (i.e., most default values).