Automerge supports deleting branch automatically after merging (#32343)

Resolve #32341 
~Depends on #27151~

- [x] It will display a checkbox of deleting the head branch on the pull
request view page when starting an auto-merge task.
- [x] Add permission check before deleting the branch
- [x] Add delete branch comment for those closing pull requests because
of head branch or base branch was deleted.
- [x] Merge `RetargetChildrenOnMerge` and `AddDeletePRBranchComment`
into `service.DeleteBranch`.

web_src/js/components/PullRequestMergeForm.vue

@@ -129,7 +129,7 @@ function clearMergeMessage() {
         {{ mergeForm.textCancel }}
       </button>
 
-      <div class="ui checkbox tw-ml-1" v-if="mergeForm.isPullBranchDeletable && !autoMergeWhenSucceed">
+      <div class="ui checkbox tw-ml-1" v-if="mergeForm.isPullBranchDeletable">
         <input name="delete_branch_after_merge" type="checkbox" v-model="deleteBranchAfterMerge" id="delete-branch-after-merge">
         <label for="delete-branch-after-merge">{{ mergeForm.textDeleteBranch }}</label>
       </div>