Previously, erroneous usage would produce error messages like:

	FAIL: testdata/script/mod_tidy_replace.txt:4: usage: stdout [-count=N] 'pattern' file

where the “file” argument is not actually valid for the stdout command.

Change-Id: I74100960f4d25da122faa6c82620995a3fbfc75f
Reviewed-on: https://go-review.googlesource.com/c/140858
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>