Add support to call ediff through emacsclient

This way Emacs can be used in Perforce, for example, as the diff program:

    Location: C:/msys64/mingw64/bin/emacsclientw.exe
    Arguments: diff %1 %2

oni-ediff.el

@@ -47,6 +47,22 @@
     (`(,fileA ,fileB ,fileC) (ediff-files3 fileA fileB fileC))
     (_ (error "Invalid number of arguments, need either 2 or 3 files"))))
 
+;;;###autoload
+(defun oni-ediff-server (fun files proc &optional nowait)
+  (pcase files
+    ((and `((,fileB) (,fileA) (,command))
+          (guard (string= (file-name-nondirectory command) "diff")))
+     (select-frame-set-input-focus (selected-frame))
+     (ediff-files (expand-file-name fileA)
+                  (expand-file-name fileB))
+     nil)
+    (_
+     (message "Got args: \"%s\"" (list files proc nowait))
+     (funcall fun files proc nowait))))
+
+;;;###autoload
+(advice-add 'server-visit-files :around #'oni-ediff-server)
+
 (setq ediff-window-setup-function 'ediff-setup-windows-plain)
 (setq ediff-split-window-function 'split-window-horizontally)