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 <pjbgf@linux.com>

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_js_test.go

@@ -7,9 +7,11 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"reflect"
 	"testing"
 
 	"github.com/go-git/go-billy/v5"
+	"github.com/go-git/go-billy/v5/memfs"
 	"github.com/go-git/go-billy/v5/test"
 
 	. "gopkg.in/check.v1"
@@ -46,3 +48,16 @@ func (s *OSSuite) TestCapabilities(c *C) {
 	caps := billy.Capabilities(s.FS)
 	c.Assert(caps, Equals, billy.DefaultCapabilities&^billy.LockCapability)
 }
+
+func TestDefault(t *testing.T) {
+	want := &memfs.Memory{}
+	got := Default
+
+	if reflect.TypeOf(got) != reflect.TypeOf(want) {
+		t.Errorf("wanted Default to be %T got %T", want, got)
+	}
+}
+
+func TestNewAPI(t *testing.T) {
+	_ = New("/")
+}

osfs/os_test.go

@@ -0,0 +1,24 @@
+//go:build !js
+// +build !js
+
+package osfs
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestDefault(t *testing.T) {
+	want := &ChrootOS{}
+	got := Default
+
+	if reflect.TypeOf(got) != reflect.TypeOf(want) {
+		t.Errorf("wanted Default to be %T got %T", want, got)
+	}
+}
+
+func TestNewAPI(t *testing.T) {
+	_ = New("/")
+	_ = New("/", WithBoundOS())
+	_ = New("/", WithChrootOS())
+}