Adding missing straight option in repositories compare api (#448)

* Adding missing straight option in repositories compare api

* Fixing struct reference for missing straight option

repositories.go

@@ -185,8 +185,9 @@ func (c Compare) String() string {
 // GitLab API docs:
 // https://docs.gitlab.com/ce/api/repositories.html#compare-branches-tags-or-commits
 type CompareOptions struct {
-	From *string `url:"from,omitempty" json:"from,omitempty"`
-	To   *string `url:"to,omitempty" json:"to,omitempty"`
+	From     *string `url:"from,omitempty" json:"from,omitempty"`
+	To       *string `url:"to,omitempty" json:"to,omitempty"`
+	Straight *bool   `url:"straight,omitempty" json:"straight,omitempty"`
 }
 
 // Compare compares branches, tags or commits.