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>

services/git/compare.go

@@ -76,7 +76,7 @@ func GetCompareInfo(ctx context.Context, baseRepo, headRepo *repo_model.Reposito
 	if !directComparison {
 		compareInfo.MergeBase, err = gitrepo.MergeBase(ctx, headRepo, compareInfo.BaseCommitID, compareInfo.HeadCommitID)
 		if err != nil {
-			return nil, fmt.Errorf("MergeBase: %w", err)
+			return compareInfo, fmt.Errorf("MergeBase: %w", err)
 		}
 	} else {
 		compareInfo.MergeBase = compareInfo.BaseCommitID