Add option to API to update PullRequest base branch (#11666)

* EditPull: add option to change base

Close #11552

modules/structs/pull.go

@@ -83,6 +83,7 @@ type CreatePullRequestOption struct {
 type EditPullRequestOption struct {
 	Title     string   `json:"title"`
 	Body      string   `json:"body"`
+	Base      string   `json:"base"`
 	Assignee  string   `json:"assignee"`
 	Assignees []string `json:"assignees"`
 	Milestone int64    `json:"milestone"`