Compare views and cleanup

- Remove compare/_head and just use commits/_head since they're identical
- Add index view; extract the form into a partial

app/views/events/_event_push.html.haml

@@ -21,6 +21,6 @@
         %li.commits-stat
           - if event.commits_count > 2
             %span ... and #{event.commits_count - 2} more commits.
-          = link_to compare_project_commits_path(event.project, from: event.parent_commit.id, to: event.last_commit.id) do
+          = link_to project_compare_path(event.project, from: event.parent_commit.id, to: event.last_commit.id) do
             %strong Compare → #{event.parent_commit.id[0..7]}...#{event.last_commit.id[0..7]}
   .clearfix