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>

options/locale/locale_en-US.json

@@ -1321,7 +1321,7 @@
   "repo.commits.desc": "Browse source code change history.",
   "repo.commits.commits": "Commits",
   "repo.commits.no_commits": "No commits in common. \"%s\" and \"%s\" have entirely different histories.",
-  "repo.commits.nothing_to_compare": "These branches are equal.",
+  "repo.commits.nothing_to_compare": "There are no differences to show.",
   "repo.commits.search.tooltip": "You can prefix keywords with \"author:\", \"committer:\", \"after:\", or \"before:\", e.g. \"revert author:Alice before:2019-01-13\".",
   "repo.commits.search_branch": "This Branch",
   "repo.commits.search_all": "All Branches",
@@ -1783,9 +1783,9 @@
   "repo.pulls.select_commit_hold_shift_for_range": "Select commit. Hold Shift and click to select a range.",
   "repo.pulls.review_only_possible_for_full_diff": "Review is only possible when viewing the full diff",
   "repo.pulls.filter_changes_by_commit": "Filter by commit",
-  "repo.pulls.nothing_to_compare": "These branches are equal. There is no need to create a pull request.",
+  "repo.pulls.nothing_to_compare": "There are no differences to show. There is no need to create a pull request.",
   "repo.pulls.no_common_history": "These branches do not share a common merge base. Select a different base or compare branch.",
-  "repo.pulls.nothing_to_compare_have_tag": "The selected branches/tags are equal.",
+  "repo.pulls.nothing_to_compare_have_tag": "There are no differences to show between the selected branches or tags.",
   "repo.pulls.nothing_to_compare_and_allow_empty_pr": "These branches are equal. This PR will be empty.",
   "repo.pulls.has_pull_request": "A pull request between these branches already exists: <a href=\"%[1]s\">%[2]s#%[3]d</a>",
   "repo.pulls.create": "Create Pull Request",

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}}