Add missing translation (go-gitea#26926)

Fix go-gitea#26923

Author: lunny

options/locale/locale_en-US.ini

@@ -1764,6 +1764,7 @@ pulls.rebase_conflict_summary = Error Message
 pulls.unrelated_histories = Merge Failed: The merge head and base do not share a common history. Hint: Try a different strategy
 pulls.merge_out_of_date = Merge Failed: Whilst generating the merge, the base was updated. Hint: Try again.
 pulls.head_out_of_date = Merge Failed: Whilst generating the merge, the head was updated. Hint: Try again.
+pulls.has_merged = Failed: The pull request has been merged, you cannot merge again or change the target branch.
 pulls.push_rejected = Merge Failed: The push was rejected. Review the Git Hooks for this repository.
 pulls.push_rejected_summary = Full Rejection Message
 pulls.push_rejected_no_message = Merge Failed: The push was rejected but there was no remote message.<br>Review the Git Hooks for this repository