Make test.File output a patch-style diff (#34)

README.md

@@ -208,6 +208,29 @@ func TestOutput(t *testing.T) {
 
 Under the hood `CaptureOutput` temporarily captures both streams, copies the data to a buffer and returns the output back to you, before cleaning everything back up again.
 
+### Golden Files
+
+`test` has great support for golden files:
+
+```go
+func TestFile(t *testing.T) {
+    got := "some contents\n"
+    want := filepath.Join(test.Data(t), "golden.txt")
+
+    test.File(t, got, want)
+}
+```
+
+This wil read the file, normalise line endings and then generate an output almost like a git diff:
+
+```patch
+--- want
++++ got
+@@ -1 +1 @@
+-some file contents
++some contents
+```
+
 ### Credits
 
 This package was created with [copier] and the [FollowTheProcess/go_copier] project template.

go.mod

@@ -3,3 +3,5 @@ module github.com/FollowTheProcess/test
 go 1.22
 
 require github.com/google/go-cmp v0.6.0
+
+require github.com/aymanbagabas/go-udiff v0.2.0

go.sum

@@ -1,2 +1,4 @@
+github.com/aymanbagabas/go-udiff v0.2.0 h1:TK0fH4MteXUDspT88n8CKzvK0X9O2xu9yQjWpi6yML8=
+github.com/aymanbagabas/go-udiff v0.2.0/go.mod h1:RE4Ex0qsGkTAJoQdQQCA0uG+nAzJO/pI/QwceO5fgrA=
 github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
 github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=

test.go

@@ -12,6 +12,7 @@ import (
 	"sync"
 	"testing"
 
+	"github.com/aymanbagabas/go-udiff"
 	"github.com/google/go-cmp/cmp"
 )
 
@@ -193,7 +194,8 @@ func Data(t testing.TB) string {
 // It takes a string and the path of a file to compare, use [Data] to obtain
 // the path to the current packages testdata directory.
 //
-// If the contents differ, the test will fail with output equivalent to [Diff].
+// If the contents differ, the test will fail with output similar to executing git diff
+// on the contents.
 //
 // Files with differing line endings (e.g windows CR LF \r\n vs unix LF \n) will be normalised to
 // \n prior to comparison so this function will behave identically across multiple platforms.
@@ -212,7 +214,9 @@ func File(t testing.TB, got, file string) {
 
 	contents = bytes.ReplaceAll(contents, []byte("\r\n"), []byte("\n"))
 
-	Diff(t, got, string(contents))
+	if diff := udiff.Unified("want", "got", string(contents), got); diff != "" {
+		t.Fatalf("\nMismatch\n--------\n%s\n", diff)
+	}
 }
 
 // CaptureOutput captures and returns data printed to stdout and stderr by the provided function fn, allowing