testscript: switch to new diff package

The main reason to prefer a copy of Go's internal/diff over pkg/diff is
that internal/diff is much more efficient in both time and memory usage.
In particular, pkg/diff required quadratic space in memory,
which could easily cause "out of memory" errors in Go tests
per pkg/diff#26.

Beyond making the `cmp` command better able to handle large files,
this also moves us back to having zero external dependencies,
which is always a nice to have.

The long_diff test still appears to work well;
the output is changed since the new package produces a shorter,
but still entirely correct, diff.

It also seems like the new package includes a leading "diff" line to
show the two filenames. That seems like a harmless change.

go.mod

@@ -1,5 +1,3 @@
 module github.com/rogpeppe/go-internal
 
 go 1.18
-
-require github.com/pkg/diff v0.0.0-20210226163009-20ebb0f2a09e

go.sum

@@ -1,2 +0,0 @@
-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=

testscript/cmd.go

@@ -16,7 +16,7 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/pkg/diff"
+	"github.com/rogpeppe/go-internal/diff"
 	"github.com/rogpeppe/go-internal/txtar"
 )
 
@@ -151,12 +151,9 @@ func (ts *TestScript) doCmdCmp(neg bool, args []string, env bool) {
 		return
 	}
 
-	var sb strings.Builder
-	if err := diff.Text(name1, name2, text1, text2, &sb); err != nil {
-		ts.Check(err)
-	}
+	unifiedDiff := diff.Diff(name1, []byte(text1), name2, []byte(text2))
 
-	ts.Logf("%s", sb.String())
+	ts.Logf("%s", unifiedDiff)
 	ts.Fatalf("%s and %s differ", name1, name2)
 }
 

testscript/testdata/long_diff.txt

@@ -111,17 +111,14 @@ cmpenv stdout stdout.golden
 >a
 -- stdout.golden --
 > cmp a b
+diff a b
 --- a
 +++ b
-@@ -1,3 +1,4 @@
+@@ -1,4 +1,4 @@
+-a
 +b
  a
  a
  a
-@@ -46,4 +47,3 @@
- a
- a
- a
--a
 
 FAIL: $$WORK${/}dir${/}script.txt:1: a and b differ