Add support for git_blob_is_binary (#625)

This adds IsBinary() func to Blob struct, which simply returns the result of git_blob_is_binary function.
Refs: https://libgit2.org/libgit2/#HEAD/group/blob/git_blob_is_binary
Issue: Add support for git_blob_is_binary #426

Fixes: #426

blob.go

@@ -40,6 +40,12 @@ func (v *Blob) Contents() []byte {
 	return goBytes
 }
 
+func (v *Blob) IsBinary() bool {
+	ret := C.git_blob_is_binary(v.cast_ptr) == 1
+	runtime.KeepAlive(v)
+	return ret
+}
+
 func (repo *Repository) CreateBlobFromBuffer(data []byte) (*Oid, error) {
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()

blob_test.go

@@ -28,7 +28,21 @@ func TestCreateBlobFromBuffer(t *testing.T) {
 		t.Fatal("Empty buffer did not deliver empty blob id")
 	}
 
-	for _, data := range []([]byte){[]byte("hello there"), doublePointerBytes()} {
+	tests := []struct {
+		data     []byte
+		isBinary bool
+	}{
+		{
+			data:     []byte("hello there"),
+			isBinary: false,
+		},
+		{
+			data:     doublePointerBytes(),
+			isBinary: true,
+		},
+	}
+	for _, tt := range tests {
+		data := tt.data
 		id, err = repo.CreateBlobFromBuffer(data)
 		checkFatal(t, err)
 
@@ -38,5 +52,9 @@ func TestCreateBlobFromBuffer(t *testing.T) {
 			t.Fatal("Loaded bytes don't match original bytes:",
 				blob.Contents(), "!=", data)
 		}
+		want := tt.isBinary
+		if got := blob.IsBinary(); got != want {
+			t.Fatalf("IsBinary() = %v, want %v", got, want)
+		}
 	}
 }