testscript: add test to show cmp with small diff between large files (#129)

Also define $ as part of the standard testscript environment, for use in
cmpenv situations where the literal $ is required in a file.

Author: myitcv

testscript/testdata/long_diff.txt

@@ -0,0 +1,167 @@
+# Verify the behaviour of cmp in the case of a small diff between
+# two large files
+
+unquote dir/script.txt
+! testscript dir
+env
+cmpenv stdout stdout.golden
+
+-- dir/script.txt --
+>cmp a b
+>
+>-- a --
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>-- b --
+>b
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+>a
+-- stdout.golden --
+
+> cmp a b
+[diff -a +b]
+-a
++b
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+ a
+
+FAIL: $$WORK${/}dir${/}script.txt:1: a and b differ

testscript/testscript.go

@@ -299,6 +299,7 @@ func (ts *TestScript) setup() string {
 			"devnull=" + os.DevNull,
 			"/=" + string(os.PathSeparator),
 			":=" + string(os.PathListSeparator),
+			"$=$",
 
 			// If we are collecting coverage profiles for merging into the main one,
 			// ensure the environment variable is forwarded to sub-processes.