Fix Dir.chdir nested with multiple absolute paths.

The previous implementation used `File.join(*paths)`, which blindly concatenates path components and should not be used for path resolution.  As a result, the following has unexpected behavior:

```ruby
# EXPECTED
#
#   Dir.pwd => "/path/to/here"
#   FakeFS::FileSystem.normalize_path('.') => "/path/to/here"
#
# ACTUAL
#
#   Dir.pwd => ""
#   FakeFS::FileSystem.normalize_path('.') => "/path/to/here/path/to/here"
#
FileUtils.mkdir_p('/path/to/here')
Dir.chdir('/path/to/here') do
  Dir.chdir('/path/to/here') do
    puts "Dir.pwd => #{Dir.pwd.inspect}"
    puts "FakeFS::FileSystem.normalize_path('.') => #{FakeFS::FileSystem.normalize_path('.').inspect}"
  end
end
```
The fix implemented here is to use `Pathname#+` to handle path resolution when there is at least one level of directories.