testscript: use unified diff for cmp-style comparisons

go.mod

@@ -2,4 +2,7 @@ module github.com/rogpeppe/go-internal
 
 go 1.11
 
-require gopkg.in/errgo.v2 v2.1.0
+require (
+	github.com/pkg/diff v0.0.0-20210226163009-20ebb0f2a09e
+	gopkg.in/errgo.v2 v2.1.0
+)

go.sum

@@ -3,6 +3,8 @@ github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORN
 github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
 github.com/kr/text v0.1.0 h1:45sCR5RtlFHMR4UwH9sdQ5TC8v0qDQCHnXt+kaKSTVE=
 github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
+github.com/pkg/diff v0.0.0-20210226163009-20ebb0f2a09e h1:aoZm08cpOy4WuID//EZDgcC4zIxODThtZNPirFr42+A=
+github.com/pkg/diff v0.0.0-20210226163009-20ebb0f2a09e/go.mod h1:pJLUxLENpZxwdsKMEsNbx1VGcRFpLqf3715MtcvvzbA=
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127 h1:qIbj1fsPNlZgppZ+VLlY7N33q108Sa+fhmuc+sWQYwY=
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/errgo.v2 v2.1.0 h1:0vLT13EuvQ0hNvakwLuFZ/jYrLp5F3kcWHXdRggjCE8=

testscript/cmd.go

@@ -16,7 +16,7 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/rogpeppe/go-internal/internal/textutil"
+	"github.com/pkg/diff"
 	"github.com/rogpeppe/go-internal/txtar"
 )
 
@@ -141,7 +141,12 @@ func (ts *TestScript) doCmdCmp(args []string, env bool) {
 		// update the script.
 	}
 
-	ts.Logf("[diff -%s +%s]\n%s\n", name1, name2, textutil.Diff(text1, text2))
+	var sb strings.Builder
+	if err := diff.Text(name1, name2, text1, text2, &sb); err != nil {
+		ts.Check(err)
+	}
+
+	ts.Logf("%s", sb.String())
 	ts.Fatalf("%s and %s differ", name1, name2)
 }
 

testscript/testdata/long_diff.txt

@@ -112,56 +112,17 @@ cmpenv stdout stdout.golden
 -- stdout.golden --
 
 > cmp a b
-[diff -a +b]
--a
+--- a
++++ b
+@@ -1,3 +1,4 @@
 +b
  a
  a
  a
+@@ -46,4 +47,3 @@
  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