New/reopen PR checks if there is any unmerged and open PR

conf/locale/locale_en-US.ini

@@ -502,6 +502,7 @@ pulls.can_auto_merge_desc = You can perform auto-merge operation on this pull re
 pulls.cannot_auto_merge_desc = You can't perform auto-merge operation because there are conflicts between commits.
 pulls.cannot_auto_merge_helper = Please use command line tool to solve it.
 pulls.merge_pull_request = Merge Pull Request
+pulls.open_unmerged_pull_exists = `You can't perform reopen operation because there is already an open pull request (#%d) from same repository with same merge information and is waiting for merging.`
 
 milestones.new = New Milestone
 milestones.open_tab = %d Open