fix GPG signature popover trigger for Safari and Firefox

app/views/projects/commit/_ajax_signature.html.haml

 - if commit.has_signature?
-  %button{ class: commit_signature_badge_classes('js-loading-gpg-badge'), data: { toggle: 'tooltip', placement: 'auto top', title: 'GPG signature (loading...)', 'commit-sha' => commit.sha } }
+  %a{ href: '#', tabindex: 0, class: commit_signature_badge_classes('js-loading-gpg-badge'), data: { toggle: 'tooltip', placement: 'auto top', title: 'GPG signature (loading...)', 'commit-sha' => commit.sha } }

app/views/projects/commit/_signature_badge.html.haml

@@ -24,5 +24,5 @@
 
   = link_to('Learn more about signing commits', help_page_path('user/project/repository/gpg_signed_commits/index.md'), class: 'gpg-popover-help-link')
 
-%button{ class: css_classes, data: { toggle: 'popover', html: 'true', placement: 'auto top', title: title, content: content } }
+%a{ href: '#', tabindex: 0, class: css_classes, data: { toggle: 'popover', html: 'true', placement: 'auto top', title: title, content: content } }
   = label