Using drush up with --gitinfofile
Update: As mentioned in the comments below, the actual answer here is that the git deploy module is required for --package-handler=git_drupalorg to work correctly.

If you just want to track the latest HEAD from some branch of some module, then you can use drush dl with --package-handler=git_drupalorg to download the file, and use git pull when you want to update your code.

drush pm-updatecode is best used in instances where you wish to track the latest stable release of the module.