Compare dropdown fails when selecting branch with no common merge-base (#37470) (#37472)

## Summary

- handle compare requests where base and head refs have no common merge
base without returning 500
- keep the compare branch selectors usable and show a clear warning
message
- add regression coverage for unrelated-history compare selection and
merge-base error detection

Fixes #37469 



Manuel Backport of: https://github.com/go-gitea/gitea/pull/37470

---------

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

routers/web/repo/pull.go

@@ -1366,6 +1366,10 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 	if ctx.Written() {
 		return
 	}
+	if ctx.Data["IsNoMergeBase"] == true {
+		ctx.JSONError(ctx.Tr("repo.pulls.no_common_history"))
+		return
+	}
 
 	validateRet := ValidateRepoMetasForNewIssue(ctx, *form, true)
 	if ctx.Written() {