windows: reset command var to sh for correct error output messages

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/4492043

src/cmd/gotest/gotest.go

@@ -285,6 +285,7 @@ func doRun(argv []string, returnStdout bool) string {
 			}
 			cmd += `"` + v + `"`
 		}
+		command = "sh"
 		argv = []string{"sh", "-c", cmd}
 	}
 	var err os.Error