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>

templates/repo/diff/compare.tmpl

@@ -13,6 +13,7 @@
 				{{ctx.Locale.Tr "action.compare_commits_general"}}
 			{{end}}
 		</h2>
+		{{template "base/alert" .}}
 		{{$BaseCompareName := $.Repository.FullName -}}
 		{{$HeadCompareName := $.HeadRepo.FullName -}}
 		{{$OwnForkCompareName := "" -}}