Add a confirm dialog for "sync fork" (#33270) (#33273)

Backport #33270 by @wxiaoguang

Try to quickly fix #33264

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

options/locale/locale_en-US.ini

@@ -1951,6 +1951,7 @@ pulls.upstream_diverging_prompt_behind_1 = This branch is %[1]d commit behind %[
 pulls.upstream_diverging_prompt_behind_n = This branch is %[1]d commits behind %[2]s
 pulls.upstream_diverging_prompt_base_newer = The base branch %s has new changes
 pulls.upstream_diverging_merge = Sync fork
+pulls.upstream_diverging_merge_confirm = Would you like to merge base repository's default branch onto this repository's branch %s?
 
 pull.deleted_branch = (deleted):%s
 pull.agit_documentation = Review documentation about AGit

templates/repo/code/upstream_diverging_info.tmpl

@@ -10,7 +10,10 @@
 		{{end}}
 	</div>
 	{{if .CanWriteCode}}
-	<button class="ui compact primary button tw-m-0 link-action" data-url="{{.Repository.Link}}/branches/merge-upstream?branch={{.BranchName}}">
+	<button class="ui compact primary button tw-m-0 link-action"
+					data-modal-confirm-header="{{ctx.Locale.Tr "repo.pulls.upstream_diverging_merge"}}"
+					data-modal-confirm-content="{{ctx.Locale.Tr "repo.pulls.upstream_diverging_merge_confirm" .BranchName}}"
+					data-url="{{.Repository.Link}}/branches/merge-upstream?branch={{.BranchName}}">
 		{{ctx.Locale.Tr "repo.pulls.upstream_diverging_merge"}}
 	</button>
 	{{end}}