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>

options/locale/locale_en-US.json

@@ -1781,6 +1781,7 @@
   "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.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_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>",