Add warning message in merge instructions when `AutodetectManualMerge` was not enabled (#31805)

not enabled

quick-f-i-x https://github.com/go-gitea/gitea/issues/31433 ? , maybe
need more disscusion about better solutions.

example view:


![image](https://github.com/user-attachments/assets/2af7e1e8-42b9-4473-89c7-12d4a9205d3f)

adtion notes about how to enable `AutodetectManualMerge`


![image](https://github.com/user-attachments/assets/28f84317-367a-40d8-b50d-a19ef7c664d4)

Signed-off-by: a1012112796 <1012112796@qq.com>

options/locale/locale_en-US.ini

@@ -1891,6 +1891,7 @@ pulls.cmd_instruction_checkout_title = Checkout
 pulls.cmd_instruction_checkout_desc = From your project repository, check out a new branch and test the changes.
 pulls.cmd_instruction_merge_title = Merge
 pulls.cmd_instruction_merge_desc = Merge the changes and update on Gitea.
+pulls.cmd_instruction_merge_warning = Warning: This operation can not merge pull request because "autodetect manual merge" was not enable
 pulls.clear_merge_message = Clear merge message
 pulls.clear_merge_message_hint = Clearing the merge message will only remove the commit message content and keep generated git trailers such as "Co-Authored-By …".
 

routers/web/repo/issue.go

@@ -1869,6 +1869,8 @@ func ViewIssue(ctx *context.Context) {
 		}
 		prConfig := prUnit.PullRequestsConfig()
 
+		ctx.Data["AutodetectManualMerge"] = prConfig.AutodetectManualMerge
+
 		var mergeStyle repo_model.MergeStyle
 		// Check correct values and select default
 		if ms, ok := ctx.Data["MergeStyle"].(repo_model.MergeStyle); !ok ||

templates/repo/issue/view_content/pull_merge_instruction.tmpl

@@ -15,7 +15,13 @@
 		<div>git checkout {{$localBranch}}</div>
 	</div>
 	{{if .ShowMergeInstructions}}
-	<div><h3>{{ctx.Locale.Tr "repo.pulls.cmd_instruction_merge_title"}}</h3>{{ctx.Locale.Tr "repo.pulls.cmd_instruction_merge_desc"}}</div>
+	<div>
+		<h3>{{ctx.Locale.Tr "repo.pulls.cmd_instruction_merge_title"}}</h3>
+		{{ctx.Locale.Tr "repo.pulls.cmd_instruction_merge_desc"}}
+		{{if not .AutodetectManualMerge}}
+			<div>{{ctx.Locale.Tr "repo.pulls.cmd_instruction_merge_warning"}}</div>
+		{{end}}
+	</div>
 	<div class="ui secondary segment">
 		<div data-pull-merge-style="merge">
 			<div>git checkout {{.PullRequest.BaseBranch}}</div>