fix(templates): avoid misleading compare message when branches lack merge base (#37651)

## 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"
/>


Fixes #37642

---------

Signed-off-by: Nicolas <bircni@icloud.com>

templates/repo/diff/compare.tmpl

@@ -210,8 +210,8 @@
 					{{template "repo/issue/new_form" .}}
 				</div>
 			{{end}}
-		{{else}}{{/* not singed-in or not for pull-request */}}
-			{{if not .CommitCount}}
+		{{else}}{{/* not signed-in or not for pull-request */}}
+			{{if and (not .CommitCount) $.CompareInfo.CompareBase}}
 				<div class="ui segment">{{ctx.Locale.Tr "repo.commits.nothing_to_compare"}}</div>
 			{{end}}
 		{{end}}