fixed tag and shit

git/tag.go

@@ -21,8 +21,10 @@ func getTag(ref string) (tag string, err error) {
 		"--tags",
 		"--abbrev=0",
 		"--always",
-		ref,
 	)
+	if ref != "" {
+		cmd.Args = append(cmd.Args, ref)
+	}
 	bts, err := cmd.CombinedOutput()
 	if err != nil {
 		return tag, errors.New(err.Error() + ": " + string(bts))

git/tag_test.go

@@ -17,4 +17,11 @@ func TestPreviousTag(t *testing.T) {
 	tag, err := PreviousTag("v0.0.1")
 	assert.NoError(err)
 	assert.NotEmpty(tag)
-}
+}
+
+func TestInvalidRef(t *testing.T) {
+	assert := assert.New(t)
+	tag, err := PreviousTag("this-should-not-exist")
+	assert.Error(err)
+	assert.Empty(tag)
+}