Fix SPC insertion being affected by previous prefix argument

After mazegaki conversion has been used once, inserting a space
character (SPC) becomes affected by the prefix argument of the
previous command, rather than the current one.

When mazegaki conversion is used for the first time, `tc-mazegaki.el`
adds a binding for SPC to `tcode-mode-map`.  Thereafter, typing SPC is
handled by `tcode-input-method`, which executes the command
`tcode-mazegaki-space-or-convert` with the value of
`current-prefix-arg` as its prefix argument.

This issue occurs when `tcode-use-input-method` is non-`nil`.
Changing this variable affects when `tcode-input-method` is invoked.
When the value is `nil`, the function is called from
`tcode-self-insert-command`, at which point `current-prefix-arg`
correctly represents the prefix argument for
`tcode-self-insert-command`, so it is appropriate to pass it along to
the subcommand.  However, when `tcode-use-input-method` is non-`nil`,
`tcode-input-method` is invoked directly from the event loop, at a
time when no command is currently executing.  In that case,
`current-prefix-arg` still holds the prefix argument from the previous
command, so using it for the subcommand is incorrect.  Use
`prefix-argument` instead when `tcode-use-input-method` is non-`nil`.

Author: tooro88

tc.el

@@ -663,8 +663,9 @@ t ... cancel"
 	 (if (commandp action)
 	     (progn
 	       ;; コマンドの実行
-	       (setq prefix-arg current-prefix-arg
-		     this-command action)
+	       (unless tcode-use-input-method
+		 (setq prefix-arg current-prefix-arg))
+	       (setq this-command action)
 	       (command-execute action))
 	   ;; コマンドでない関数の実行
 	   (funcall action)