support sorting multi-file diff before printing

Makes it easier to canonicalize the output of `PrintMultiFileDiff` by adding a new `PrintOptions` arg that lets the caller specify that the diffs should be sorted before printing.

Author: sqs

diff/diff_test.go

@@ -510,7 +510,7 @@ func TestParseMultiFileDiffAndPrintMultiFileDiff(t *testing.T) {
 			t.Errorf("%s: got %v instances of diff.FileDiff, expected %v", test.filename, got, want)
 		}
 
-		printed, err := PrintMultiFileDiff(diffs)
+		printed, err := PrintMultiFileDiff(diffs, nil)
 		if err != nil {
 			t.Errorf("%s: PrintMultiFileDiff: %s", test.filename, err)
 		}

diff/print.go

@@ -4,13 +4,25 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"sort"
 	"time"
 
 	"sourcegraph.com/sqs/pbtypes"
 )
 
-// PrintMultiFileDiff prints a multi-file diff in unified diff format.
-func PrintMultiFileDiff(ds []*FileDiff) ([]byte, error) {
+// PrintOptions specifies how to print a diff.
+type PrintOptions struct {
+	Sort bool // sort by filename
+}
+
+// PrintMultiFileDiff prints a multi-file diff in unified diff format with the specified options.
+func PrintMultiFileDiff(ds []*FileDiff, opt *PrintOptions) ([]byte, error) {
+	if opt != nil && opt.Sort {
+		sort.Slice(ds, func(i, j int) bool {
+			return ds[i].NewName < ds[j].NewName
+		})
+	}
+
 	var buf bytes.Buffer
 	for _, d := range ds {
 		diff, err := PrintFileDiff(d)