tree: add IsLink method for TreeEntry class

Some callers may need to disambiguate between regular blob
objects and those which represent symbolic links, so we add
an IsLink() method to the TreeEntry class that returns true
when the object is a blob and a symlink, and false otherwise.

We also expand the test suite slightly to exercise this new
method.

Author: chrisd8088

tree.go

@@ -234,6 +234,12 @@ func (e *TreeEntry) Type() ObjectType {
 	}
 }
 
+// IsLink returns true if the given TreeEntry is a blob which represents a
+// symbolic link (i.e., with a filemode of 0120000.
+func (e *TreeEntry) IsLink() bool {
+	return e.Filemode & sIFMT == sIFLNK
+}
+
 // SubtreeOrder is an implementation of sort.Interface that sorts a set of
 // `*TreeEntry`'s according to "subtree" order. This ordering is required to
 // write trees in a correct, readable format to the Git object database.

tree_test.go

@@ -166,9 +166,10 @@ func TestMergeInsertElementsInSubtreeOrder(t *testing.T) {
 type TreeEntryTypeTestCase struct {
 	Filemode int32
 	Expected ObjectType
+	IsLink bool
 }
 
-func (c *TreeEntryTypeTestCase) Assert(t *testing.T) {
+func (c *TreeEntryTypeTestCase) AssertType(t *testing.T) {
 	e := &TreeEntry{Filemode: c.Filemode}
 
 	got := e.Type()
@@ -177,14 +178,24 @@ func (c *TreeEntryTypeTestCase) Assert(t *testing.T) {
 		"gitobj: expected type: %s, got: %s", c.Expected, got)
 }
 
+func (c *TreeEntryTypeTestCase) AssertIsLink(t *testing.T) {
+	e := &TreeEntry{Filemode: c.Filemode}
+
+	isLink := e.IsLink()
+
+	assert.Equal(t, c.IsLink, isLink,
+		"gitobj: expected link: %v, got: %v, for type %s", c.IsLink, isLink, c.Expected)
+}
+
 func TestTreeEntryTypeResolution(t *testing.T) {
 	for desc, c := range map[string]*TreeEntryTypeTestCase{
-		"blob":    {0100644, BlobObjectType},
-		"subtree": {040000, TreeObjectType},
-		"symlink": {0120000, BlobObjectType},
-		"commit":  {0160000, CommitObjectType},
+		"blob":    {0100644, BlobObjectType, false},
+		"subtree": {040000, TreeObjectType, false},
+		"symlink": {0120000, BlobObjectType, true},
+		"commit":  {0160000, CommitObjectType, false},
 	} {
-		t.Run(desc, c.Assert)
+		t.Run(desc, c.AssertType)
+		t.Run(desc, c.AssertIsLink)
 	}
 }