Update src-d/go-git to v4.13.0 (#7688)

* update gopkg.in/src-d/go-git.v4 v4.13.0

* mod tidy

* vendor

vendor/golang.org/x/sys/unix/readdirent_getdents.go

@@ -0,0 +1,12 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build aix dragonfly freebsd linux netbsd openbsd
+
+package unix
+
+// ReadDirent reads directory entries from fd and writes them into buf.
+func ReadDirent(fd int, buf []byte) (n int, err error) {
+	return Getdents(fd, buf)
+}