RepoAssignment ensure to close before overwrite (#19449)

* check if GitRepo already open and close if

* only run RepoAssignment once

* refactor context helper for api to open GitRepo

routers/api/v1/repo/hook.go

@@ -138,6 +138,11 @@ func TestHook(ctx *context.APIContext) {
 	//   type: integer
 	//   format: int64
 	//   required: true
+	// - name: ref
+	//   in: query
+	//   description: "The name of the commit/branch/tag. Default the repository’s default branch (usually master)"
+	//   type: string
+	//   required: false
 	// responses:
 	//   "204":
 	//     "$ref": "#/responses/empty"