Re-introduce osfs.Default

The Default var was incorrectly removed as part of
go-git#31. This PR revert that change and adds tests to
avoid future regression.

Signed-off-by: Paulo Gomes <[email protected]>

Author: pjbgf

osfs/os.go

@@ -18,6 +18,9 @@ const (
 	defaultCreateMode    = 0o666
 )
 
+// Default Filesystem representing the root of the os filesystem.
+var Default = &ChrootOS{}
+
 // New returns a new OS filesystem.
 func New(baseDir string, opts ...Option) billy.Filesystem {
 	o := &options{}

osfs/os_test.go

@@ -0,0 +1,18 @@
+package osfs
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestDefault(t *testing.T) {
+	if reflect.TypeOf(Default) != reflect.TypeOf(&ChrootOS{}) {
+		t.Errorf("wanted Default to be %T got %T", ChrootOS{}, Default)
+	}
+}
+
+func TestNewAPI(t *testing.T) {
+	_ = New("/")
+	_ = New("/", WithBoundOS())
+	_ = New("/", WithChrootOS())
+}