Add Commit#==

Prior, comparison would use the Ruby object's ID, which got out of sync
after a Spring fork and would result in erroneous test failures.

Now we just check that the compared object is a Commit and then compare
their underlying raw commit objects.

app/models/commit.rb

@@ -56,6 +56,10 @@ class Commit
     @raw.id
   end
 
+  def ==(other)
+    (self.class === other) && (raw == other.raw)
+  end
+
   def diff_line_count
     @diff_line_count ||= Commit::diff_line_count(self.diffs)
     @diff_line_count