Merge branch 'handle-bad-head' into 'master'

Fix Error 500 occuring with repositories that have a bad HEAD

A repository could have objects but no valid HEAD, causing `project.commit` to be `nil`.

See merge request !1433

app/views/shared/projects/_project.html.haml

@@ -18,7 +18,7 @@
           = project.name
 
     .project-controls
-      - if ci && !project.empty_repo?
+      - if ci && !project.empty_repo? && project.commit
         - if ci_commit = project.ci_commit(project.commit.sha)
           = link_to ci_status_path(ci_commit), class: "c#{ci_status_color(ci_commit)}",
             title: "Build status: #{ci_commit.status}", data: {toggle: 'tooltip', placement: 'left'} do