updateDiffSync(): fix potential race

updateDiffSync() may called asynchronously, so 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

@@ -1291,7 +1291,12 @@ func (b *Buffer) updateDiffSync() {
 	}
 
 	differ := dmp.New()
-	baseRunes, bufferRunes, _ := differ.DiffLinesToRunes(string(b.diffBase), string(b.Bytes()))
+
+	b.Lock()
+	bytes := b.Bytes()
+	b.Unlock()
+
+	baseRunes, bufferRunes, _ := differ.DiffLinesToRunes(string(b.diffBase), string(bytes))
 	diffs := differ.DiffMainRunes(baseRunes, bufferRunes, false)
 	lineN := 0