Compare ids of commits if present for equality test

This solves a problem where commits populated with Gitaly were not equal
to commits populated with Rugged. This is because Gitaly may not return
all fields of a commit for optimizations purposes, which resulted in
false negatives when comparing the same commit (commits with the same
sha) with different sources.

lib/gitlab/git/commit.rb

@@ -19,13 +19,7 @@ module Gitlab
       def ==(other)
         return false unless other.is_a?(Gitlab::Git::Commit)
 
-        methods = [:message, :parent_ids, :authored_date, :author_name,
-                   :author_email, :committed_date, :committer_name,
-                   :committer_email]
-
-        methods.all? do |method|
-          send(method) == other.send(method)
-        end
+        id && id == other.id
       end
 
       class << self