Memory.ReadDir() should return an error when path isn't found.

Presently, it returns an empty list of files and a `nil` error value.
This PR aims to fix go-git#37

memfs/memory.go

@@ -9,6 +9,7 @@ import (
 	"path/filepath"
 	"sort"
 	"strings"
+	"syscall"
 	"time"
 
 	"github.com/go-git/go-billy/v5"
@@ -25,7 +26,7 @@ type Memory struct {
 	tempCount int
 }
 
-//New returns a new Memory filesystem.
+// New returns a new Memory filesystem.
 func New() billy.Filesystem {
 	fs := &Memory{s: newStorage()}
 	return chroot.New(fs, string(separator))
@@ -133,6 +134,8 @@ func (fs *Memory) ReadDir(path string) ([]os.FileInfo, error) {
 		if target, isLink := fs.resolveLink(path, f); isLink {
 			return fs.ReadDir(target)
 		}
+	} else {
+		return nil, &os.PathError{Op: "open", Path: path, Err: syscall.ENOENT}
 	}
 
 	var entries []os.FileInfo

memfs/memory_test.go

@@ -85,6 +85,12 @@ func (s *MemorySuite) TestOrder(c *C) {
 	}
 }
 
+func (s *MemorySuite) TestNotFound(c *C) {
+	files, err := s.FS.ReadDir("asdf")
+	c.Assert(files, HasLen, 0)
+	c.Assert(err, ErrorMatches, "open /asdf: no such file or directory")
+}
+
 func (s *MemorySuite) TestTruncateAppend(c *C) {
 	err := util.WriteFile(s.FS, "truncate_append", []byte("file-content"), 0666)
 	c.Assert(err, IsNil)