sh.do: explicitly check /bin/sh instead of sh.

Because our experiment involves creating a file called redo/sh, and the
redo directory (or some other version of redo's dirctory) might be on
the path, just testing 'sh' is actually wrong: we might end up with
some earlier version of redo-sh.

Instead, specifically for sh, always demand that we test /bin/sh.

redo-sh.do

@@ -12,7 +12,7 @@ WARN=
 # pass the tests, because weaker shells are more likely to point out when you
 # use some non-portable feature.
 for sh in dash /usr/xpg4/bin/sh ash posh mksh ksh ksh88 ksh93 pdksh \
-		zsh bash busybox sh; do
+		zsh bash busybox /bin/sh; do
 	printf "%-30s" "Testing $sh..."
 	FOUND=`which $sh 2>/dev/null` || { echo "missing"; continue; }