Update logic for validates_merge_request for tree of MR

For case:

Merge request №1: branch "feature" to "release" and remove "feature" branch

Merge request №2: branch "release" to "master" and remove "release" branch

Merge request №1 - broken :(

app/controllers/projects/merge_requests_controller.rb

@@ -122,12 +122,16 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   end
 
   def validates_merge_request
-    # Show git not found page if target branch doesn't exist
-    return invalid_mr unless @project.repository.branch_names.include?(@merge_request.target_branch)
-
-    # Show git not found page if source branch doesn't exist
-    # and there is no saved commits between source & target branch
-    return invalid_mr if !@project.repository.branch_names.include?(@merge_request.source_branch) && @merge_request.commits.blank?
+    # Show git not found page
+    # if there is no saved commits between source & target branch
+    if @merge_request.commits.blank?
+       # and
+       # if source target doesn't exist
+       return invalid_mr if !@project.repository.branch_names.include?(@merge_request.target_branch)
+
+       # or if source branch doesn't exist
+       return invalid_mr if !@project.repository.branch_names.include?(@merge_request.source_branch)
+    end
   end
 
   def define_show_vars