Run Ediff by patching the buffer specified at prompt.
Without the optional prefix ARG, asks if the patch is in some buffer and
prompts for the buffer or a file, depending on the answer.
With ARG=1, assumes the patch is in a file and prompts for the file.
With ARG=2, assumes the patch is in a buffer and prompts for the buffer.
PATCH-BUF is an optional argument, which specifies the buffer that contains the
patch. If not given, the user is prompted according to the prefix argument.
(fn &optional ARG PATCH-BUF)