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