Refactor create_params and update_params

Let's move shared code between create_params and update_params
into a new base_params.

update_params becomes very thin, but it still may be clearer that
the params are being shared if we have a method called
base_params, rather than have create_params merge in
update_params.

app/services/merge_requests/push_options_handler_service.rb

@@ -117,14 +117,8 @@ module MergeRequests
       collect_errors_from_merge_request(merge_request) unless merge_request.valid?
     end
 
-    def create_params(branch)
-      params = {
-        assignees: [current_user],
-        source_branch: branch,
-        source_project: project,
-        target_branch: push_options[:target] || target_project.default_branch,
-        target_project: target_project
-      }
+    def base_params
+      params = {}
 
       if push_options.key?(:merge_when_pipeline_succeeds)
         params.merge!(
@@ -137,28 +131,30 @@ module MergeRequests
         params[:force_remove_source_branch] = push_options[:remove_source_branch]
       end
 
+      if push_options.key?(:target)
+        params[:target_branch] = push_options[:target]
+      end
+
       params
     end
 
-    def update_params
-      params = {}
+    def create_params(branch)
+      params = base_params
 
-      if push_options.key?(:merge_when_pipeline_succeeds)
       params.merge!(
-          merge_when_pipeline_succeeds: push_options[:merge_when_pipeline_succeeds],
-          merge_user: current_user
+        assignees: [current_user],
+        source_branch: branch,
+        source_project: project,
+        target_project: target_project
       )
-      end
 
-      if push_options.key?(:remove_source_branch)
-        params[:force_remove_source_branch] = push_options[:remove_source_branch]
-      end
+      params[:target_branch] ||= target_project.default_branch
 
-      if push_options.key?(:target)
-        params[:target_branch] = push_options[:target]
+      params
     end
 
-      params
+    def update_params
+      base_params
     end
 
     def collect_errors_from_merge_request(merge_request)