Use pulls in commit graph unless pulls are disabled (#15774)

Fix #15370

Signed-off-by: Andrew Thornton <art27@cantab.net>

templates/repo/graph/commits.tmpl

@@ -35,7 +35,7 @@
 							{{if eq $refGroup "pull"}}
 								{{if or (not $.HidePRRefs) (containGeneric $.SelectedBranches .Name)}}
 									<!-- it's intended to use issues not pulls, if it's a pull you will get redirected -->
-									<a class="ui labelled icon button basic tiny" href="{{$.RepoLink}}/issues/{{.ShortName|PathEscape}}">
+									<a class="ui labelled icon button basic tiny" href="{{$.RepoLink}}/{{if $.Repository.UnitEnabled $.UnitTypePullRequests}}pulls{{else}}issues{{end}}/{{.ShortName|PathEscape}}">
 										{{svg "octicon-git-pull-request" 16 "mr-2"}}#{{.ShortName}}
 									</a>
 								{{end}}