Add subroutine for common parse call code

Borrowed from tj#1917 and tj#1919.

lib/command.js

@@ -897,6 +897,21 @@ Expecting one of '${allowedValues.join("', '")}'`);
     return userArgs;
   }
 
+  /**
+   * @param {boolean} async
+   * @param {Function} userArgsCallback
+   * @param {string[]} [argv]
+   * @param {Object} [parseOptions]
+   * @param {string} [parseOptions.from]
+   * @return {Command|Promise}
+   * @api private
+   */
+
+  _parseSubroutine(async, userArgsCallback, argv, parseOptions) {
+    const userArgs = this._prepareUserArgs(argv, parseOptions);
+    return userArgsCallback(userArgs);
+  }
+
   /**
    * Parse `argv`, setting options and invoking commands when defined.
    *
@@ -915,10 +930,10 @@ Expecting one of '${allowedValues.join("', '")}'`);
    */
 
   parse(argv, parseOptions) {
-    const userArgs = this._prepareUserArgs(argv, parseOptions);
-    this._parseCommand([], userArgs);
-
-    return this;
+    return this._parseSubroutine(false, (userArgs) => {
+      this._parseCommand([], userArgs);
+      return this;
+    }, argv, parseOptions);
   }
 
   /**
@@ -941,10 +956,10 @@ Expecting one of '${allowedValues.join("', '")}'`);
    */
 
   async parseAsync(argv, parseOptions) {
-    const userArgs = this._prepareUserArgs(argv, parseOptions);
-    await this._parseCommand([], userArgs);
-
-    return this;
+    return this._parseSubroutine(true, async(userArgs) => {
+      await this._parseCommand([], userArgs);
+      return this;
+    }, argv, parseOptions);
   }
 
   /**