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

Before:

![image](https://github.com/go-gitea/gitea/assets/18380374/ba69252f-3582-414a-9d62-b2b78d7035dd)
After:

![image](https://github.com/go-gitea/gitea/assets/18380374/71b8587c-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.

https://github.com/go-gitea/gitea/blob/a370efc13f0e1ea309e324639832832bc14cb6dc/templates/repo/diff/compare.tmpl#L2
But in the js, we are using `.repository.compare.pull` to find the
button:

https://github.com/go-gitea/gitea/blob/a370efc13f0e1ea309e324639832832bc14cb6dc/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

templates/repo/diff/compare.tmpl

@@ -177,16 +177,18 @@
 	</div>
 
 	{{if .IsNothingToCompare}}
-		{{if and $.IsSigned $.AllowEmptyPr (not .Repository.IsArchived)}}
+		{{if and $.IsSigned $.AllowEmptyPr (not .Repository.IsArchived) .PageIsComparePull}}
 			<div class="ui segment">{{ctx.Locale.Tr "repo.pulls.nothing_to_compare_and_allow_empty_pr"}}</div>
 			<div class="ui info message show-form-container {{if .Flash}}gt-hidden{{end}}">
 				<button class="ui button primary show-form">{{ctx.Locale.Tr "repo.pulls.new"}}</button>
 			</div>
 			<div class="pullrequest-form {{if not .Flash}}gt-hidden{{end}}">
 				{{template "repo/issue/new_form" .}}
 			</div>
-		{{else}}
+		{{else if and .HeadIsBranch .BaseIsBranch}}
 			<div class="ui segment">{{ctx.Locale.Tr "repo.pulls.nothing_to_compare"}}</div>
+		{{else}}
+			<div class="ui segment">{{ctx.Locale.Tr "repo.pulls.nothing_to_compare_have_tag"}}</div>
 		{{end}}
 	{{else if and .PageIsComparePull (gt .CommitCount 0)}}
 		{{if .HasPullRequest}}
@@ -229,7 +231,7 @@
 			{{end}}
 			{{$showDiffBox = true}}
 		{{end}}
-	{{else}}
+	{{else if not .IsNothingToCompare}}
 		{{$showDiffBox = true}}
 	{{end}}
 	</div>