Walker returns incorrect results after upgrade from 0.25.0b9 to 0.25.0b10

This is probably related to (https://github.com/libgit2/rugged/issues/663 /cc @cfillion).

Scenario:

Given a repository:

* commit C (HEAD)
|
* commit B
|
* commit A

When we do:

walker.push sha_of_a
walker.hide sha_of_c

Walker returns 1 commit (commit A) instead of 0 commits. This happens only when using Rugged::SORT_DATE.

Script to reproduce

gem 'rugged', "= #{ENV['RUGGED_VERSION']}"
require 'rugged'
require 'tmpdir'

Class.new do
  def initialize
    path = Dir.mktmpdir 'test-repository'
    @repo = Rugged::Repository.init_at path
    @repo.config['user.name'] = 'John Doe'
    @repo.config['user.email'] = 'john@doe.com'

    @hash_a = create_commit
    sleep 1
    @hash_b = create_commit
    sleep 1
    @hash_c = create_commit

    puts Rugged::VERSION
    compare
  end

  def create_commit
    Rugged::Commit.create @repo,
      message: 'Hello World',
      parents: @repo.empty? ? [] : [@repo.head.target].compact,
      tree: @repo.index.write_tree(@repo),
      update_ref: 'HEAD'
  end

  def compare
    walker = Rugged::Walker.new @repo
    walker.sorting(Rugged::SORT_DATE)

    walker.push @hash_a
    walker.hide @hash_c

    commits = walker.to_a
    walker.reset

    puts commits.size
  end
end.new

> RUGGED_VERSION=0.25.0b9 ruby rugged.rb
0.25.0b9
0
> RUGGED_VERSION=0.25.0b10 ruby rugged.rb
0.25.0b10
1

Start with the reproduction script in the issue and run it against Rugged 0.25.0b9 and 0.25.0b10. Trace the Rugged::Walker path used by SORT_DATE, focusing on push and hide for the A-to-C history. Done means the scenario returns zero commits under 0.25.0b10 while preserving the expected behavior for the earlier version.