Fix the issue where rg.el couldn't search Unicode.

The reason of rg.el wasn't able to search Unicode character on Windows
is because at the moment NTEmacs limits non-ASCII file arguments to
the current codepage. See
https://github.com/emacs-mirror/emacs/blob/58a7b99823c5c42161e9acf2abf6c22afd4da4cd/src/w32.c#L1648.

This patch provides a workaround: Instead of passing Unicode arguments
to ripgrep via Emacs, it via a temp .bat script, which was generated
whenever `rg-build-command`. This allows rg.el now to search the
entire Unicode planes, rather than being restricted to a specific
codepage.

P.s. This feature is disabled by default for keeping old behavior,
enable it via set rg-w32-unicode = t.

rg.el

@@ -144,6 +144,19 @@ Disabling this setting can break functionality of this package."
   :type 'boolean
   :group 'rg)
 
+(defcustom rg-w32-unicode nil
+  "Enable Unicode support on Windows.
+A workaround for NTEmacs subprocess not supporting Unicode arguments."
+  :type 'boolean
+  :group 'rg)
+
+(defcustom rg-w32-ripgrep-proxy
+  (expand-file-name "rg-w32-ripgrep-proxy.bat" user-emacs-directory)
+  "An automatically generated temporary batch file.
+Used to proxy ripgrep Unicode arguments."
+  :type 'string
+  :group 'rg)
+
 ;;;###autoload
 (defvar rg-command-line-flags-function 'identity
   "Function to modify command line flags of a search.
@@ -297,10 +310,19 @@ are command line flags to use for the search."
           (when (member system-type '(darwin windows-nt))
             (list ".")))))
 
-    (grep-expand-template
-     (mapconcat 'identity (cons (rg-executable) (delete-dups command-line)) " ")
-     pattern
-     (if (rg-is-custom-file-pattern files) "custom" files))))
+    (let ((command (grep-expand-template
+                    (mapconcat 'identity (cons (rg-executable) (delete-dups command-line)) " ")
+                    pattern
+                    (if (rg-is-custom-file-pattern files) "custom" files))))
+      (cond ((and (eq system-type 'windows-nt) rg-w32-unicode)
+             (with-temp-file rg-w32-ripgrep-proxy
+               (set-buffer-multibyte t)
+               (setq buffer-file-coding-system 'utf-8-dos)
+               (insert (format "@echo off\n"))
+               (insert (format "chcp 65001 > nul\n"))
+               (insert (format "%s\n" command)))
+             rg-w32-ripgrep-proxy)
+            (t command)))))
 
 (defun rg-invoke-rg-type-list ()
   "Invokes rg --type-list and return the result."