improve the compare page (#36261)

- The compare page head title should be `compare` but not `new pull
request`.
- Use `UnstableGuessRefByShortName` instead of duplicated functions
calls.
- Direct-compare, tags, commits compare will not display `New Pull
Request` button any more.

The new screenshot
<img width="1459" height="391" alt="image"
src="https://github.com/user-attachments/assets/64e9b070-9c0b-41d1-b4b8-233b96270e1b"
/>

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

options/locale/locale_en-US.json

@@ -1736,8 +1736,11 @@
   "repo.issues.reference_link": "Reference: %s",
   "repo.compare.compare_base": "base",
   "repo.compare.compare_head": "compare",
+  "repo.compare.title": "Comparing changes",
+  "repo.compare.description": "Choose two branches or tags to see what’s changed or to start a new pull request.",
   "repo.pulls.desc": "Enable pull requests and code reviews.",
   "repo.pulls.new": "New Pull Request",
+  "repo.pulls.new.description": "Discuss and review the changes in this comparison with others.",
   "repo.pulls.new.blocked_user": "Cannot create pull request because you are blocked by the repository owner.",
   "repo.pulls.new.must_collaborator": "You must be a collaborator to create pull request.",
   "repo.pulls.new.already_existed": "A pull request between these branches already exists",
@@ -1747,7 +1750,6 @@
   "repo.pulls.allow_edits_from_maintainers": "Allow edits from maintainers",
   "repo.pulls.allow_edits_from_maintainers_desc": "Users with write access to the base branch can also push to this branch",
   "repo.pulls.allow_edits_from_maintainers_err": "Updating failed",
-  "repo.pulls.compare_changes_desc": "Select the branch to merge into and the branch to pull from.",
   "repo.pulls.has_viewed_file": "Viewed",
   "repo.pulls.has_changed_since_last_review": "Changed since your last review",
   "repo.pulls.viewed_files_label": "%[1]d / %[2]d files viewed",