Don't show new pr button when page is not compare pull (#26431)

Before:

![image](ba69252f-3582-414a-9d62-b2b78d7035dd)
After:

![image](71b8587c-b96e-48fb-b3e5-1a2a8b5d06fa)
(TestOrg:test is a tag not branch)

Problem:
In the template, we will not add `compare pull` class when
`PageIsComparePull` is false.

a370efc13f/templates/repo/diff/compare.tmpl (L2)
But in the js, we are using `.repository.compare.pull` to find the
button:

a370efc13f/web_src/js/features/repo-legacy.js (L552-L563)
So, if `PageIsComparePull` is false, the `New Pull Request` button will
be there, but has no response when we click it.

options/locale/locale_en-US.ini

@@ -1714,6 +1714,7 @@ pulls.select_commit_hold_shift_for_range = Select commit. Hold shift + click to
 pulls.review_only_possible_for_full_diff = Review is only possible when viewing the full diff
 pulls.filter_changes_by_commit = Filter by commit
 pulls.nothing_to_compare = These branches are equal. There is no need to create a pull request.
+pulls.nothing_to_compare_have_tag = The selected branch/tag are equal.
 pulls.nothing_to_compare_and_allow_empty_pr = These branches are equal. This PR will be empty.
 pulls.has_pull_request = `A pull request between these branches already exists: <a href="%[1]s">%[2]s#%[3]d</a>`
 pulls.create = Create Pull Request