Add test from #612 for usage error Argument format

Adds a test that ensures that when an Argument is formatted into a usage
error, its metavar is used, not its name.

tests/test_formatting.py

@@ -163,6 +163,26 @@ def cmd(arg):
     ]
 
 
+def test_formatting_usage_error_metavar(runner):
+    """
+    :author: @r-m-n
+    Including attribution to #612
+    """
+    @click.command()
+    @click.argument('arg', metavar='metavar')
+    def cmd(arg):
+        pass
+
+    result = runner.invoke(cmd, [])
+    assert result.exit_code == 2
+    assert result.output.splitlines() == [
+        'Usage: cmd [OPTIONS] metavar',
+        'Try "cmd --help" for help.',
+        '',
+        'Error: Missing argument "metavar".'
+    ]
+
+
 def test_formatting_usage_error_nested(runner):
     @click.group()
     def cmd():