[API] Extend contents with dates (#9464)

* extend CommitTree func

* make sure Date NOT nil

* spell corection

Co-Authored-By: zeripath <art27@cantab.net>

* add TEST

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/api/v1/swagger/options.go

@@ -118,6 +118,9 @@ type swaggerParameterBodies struct {
 	// in:body
 	DeleteFileOptions api.DeleteFileOptions
 
+	// in:body
+	CommitDateOptions api.CommitDateOptions
+
 	// in:body
 	RepoTopicOptions api.RepoTopicOptions
 }