Discolor pull request tab labels (#23950) (#23987)

Backport #23950 by @silverwind

#22687 has discolored the repo tab
labels. This does the same for PR tabs for consistency. Value `0` is
still rendered like before.

Before:
<img width="502" alt="Screenshot 2023-04-06 at 19 35 24"
src="https://user-images.githubusercontent.com/115237/230454329-db6244ff-7d7e-4a2f-9240-f618a1c57f8c.png">

After:
<img width="497" alt="Screenshot 2023-04-06 at 19 35 07"
src="https://user-images.githubusercontent.com/115237/230454321-a0be6551-8c31-45e4-a1fb-ffc0d85d87bf.png">
<img width="497" alt="Screenshot 2023-04-06 at 19 38 37"
src="https://user-images.githubusercontent.com/115237/230454570-2fe2aa67-dfa7-4b2e-8c04-8dd40e6e99fc.png">

Co-authored-by: silverwind <me@silverwind.io>

templates/repo/pulls/tab_menu.tmpl

@@ -2,16 +2,16 @@
 	<a class="item {{if .PageIsPullConversation}}active{{end}}" href="{{.Issue.Link}}">
 		{{svg "octicon-comment-discussion"}}
 		{{$.locale.Tr "repo.pulls.tab_conversation"}}
-		<span class="ui {{if not .Issue.NumComments}}gray{{else}}primary{{end}} small label">{{.Issue.NumComments}}</span>
+		<span class="ui small label">{{.Issue.NumComments}}</span>
 	</a>
 	<a class="item {{if .PageIsPullCommits}}active{{end}}" {{if .NumCommits}}href="{{.Issue.Link}}/commits"{{end}}>
 		{{svg "octicon-git-commit"}}
 		{{$.locale.Tr "repo.pulls.tab_commits"}}
-		<span class="ui {{if not .NumCommits}}gray{{else}}primary{{end}} small label">{{if .NumCommits}}{{.NumCommits}}{{else}}N/A{{end}}</span>
+		<span class="ui small label">{{if .NumCommits}}{{.NumCommits}}{{else}}N/A{{end}}</span>
 	</a>
 	<a class="item {{if .PageIsPullFiles}}active{{end}}" {{if .NumFiles}}href="{{.Issue.Link}}/files"{{end}}>
 		{{svg "octicon-diff"}}
 		{{$.locale.Tr "repo.pulls.tab_files"}}
-		<span class="ui {{if not .NumFiles}}gray{{else}}primary{{end}} small label">{{if .NumFiles}}{{.NumFiles}}{{else}}N/A{{end}}</span>
+		<span class="ui small label">{{if .NumFiles}}{{.NumFiles}}{{else}}N/A{{end}}</span>
 	</a>
 </div>