'-' is one of shell special parameters.

The existing implementation of isShellSpecialVar missed '-'
from the list, causing "$-" and "${-}" expand differently.

Fixes #16554

Change-Id: Iafc7984692cc83cff58f7c1e01267bf78b3a20a9
Reviewed-on: https://go-review.googlesource.com/25352Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>