fix show correct mergebase (#37656)

## Summary

When comparing branches with **no common merge base** (e.g. unrelated
histories or orphan branches), `PageIsComparePull` is false and
`CommitCount` is zero. The compare template still showed
`repo.commits.nothing_to_compare`, which in German reads like the
branches are identical—even though the flash already explains there is
no merge base.

## Changes

- **`templates/repo/diff/compare.tmpl`**: Only render the grey “nothing
to compare” segment when `CompareInfo.CompareBase` is set.

<img width="1962" height="564"
src="https://github.com/user-attachments/assets/adc3b4a0-6f03-45da-b297-e15e5ad0aa79"
/>

---

Backport of https://github.com/go-gitea/gitea/pull/37651

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

templates/repo/diff/compare.tmpl

@@ -211,7 +211,7 @@
 				</div>
 			{{end}}
 		{{else}}{{/* not singed-in or not for pull-request */}}
-			{{if not .CommitCount}}
+			{{if and (not .CommitCount) $.CompareInfo.MergeBase}}
 				<div class="ui segment">{{ctx.Locale.Tr "repo.commits.nothing_to_compare"}}</div>
 			{{end}}
 		{{end}}