updateDiffSync(): fix potential race

When updateDiffSync() is called asynchronously, it should lock the
line array when calling Bytes(), to prevent race if the line array is
being modified by the main goroutine in the meantime.

internal/buffer/buffer.go

@@ -1280,7 +1280,7 @@ func (b *Buffer) Write(bytes []byte) (n int, err error) {
 	return len(bytes), nil
 }
 
-func (b *Buffer) updateDiffSync() {
+func (b *Buffer) updateDiff(synchronous bool) {
 	b.diffLock.Lock()
 	defer b.diffLock.Unlock()
 
@@ -1291,7 +1291,16 @@ func (b *Buffer) updateDiffSync() {
 	}
 
 	differ := dmp.New()
-	baseRunes, bufferRunes, _ := differ.DiffLinesToRunes(string(b.diffBase), string(b.Bytes()))
+
+	if !synchronous {
+		b.Lock()
+	}
+	bytes := b.Bytes()
+	if !synchronous {
+		b.Unlock()
+	}
+
+	baseRunes, bufferRunes, _ := differ.DiffLinesToRunes(string(b.diffBase), string(bytes))
 	diffs := differ.DiffMainRunes(baseRunes, bufferRunes, false)
 	lineN := 0
 
@@ -1333,11 +1342,11 @@ func (b *Buffer) UpdateDiff() {
 	}
 
 	if lineCount < 1000 {
-		b.updateDiffSync()
+		b.updateDiff(true)
 	} else if lineCount < 30000 {
 		b.updateDiffTimer = time.AfterFunc(500*time.Millisecond, func() {
 			b.updateDiffTimer = nil
-			b.updateDiffSync()
+			b.updateDiff(false)
 			screen.Redraw()
 		})
 	} else {