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_getdirentries.go

@@ -0,0 +1,19 @@
+// 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 darwin
+
+package unix
+
+import "unsafe"
+
+// ReadDirent reads directory entries from fd and writes them into buf.
+func ReadDirent(fd int, buf []byte) (n int, err error) {
+	// Final argument is (basep *uintptr) and the syscall doesn't take nil.
+	// 64 bits should be enough. (32 bits isn't even on 386). Since the
+	// actual system call is getdirentries64, 64 is a good guess.
+	// TODO(rsc): Can we use a single global basep for all calls?
+	var base = (*uintptr)(unsafe.Pointer(new(uint64)))
+	return Getdirentries(fd, buf, base)
+}