Using the 'revision' shortcut doesn't work if a branch or other download parameters are specified

Back before Drush 5.x was released, a shortcut to specify a git revision at the top level (rather than in the download section) was introduced:

projects[foo][revision] = "1234567"

is a shortcut for

projects[foo][download][revision] = "1234567"

However, there is a bug in the logic where this only works if no other download parameters are specified.

    // If download components are specified, but not the download
    // type, default to git. Additionally, if the 'revision' parameter is
    // passed at the top level, this is short-hand for download revision.
    if (isset($project['revision']) && !isset($project['download'])) {
      $project['download']['revision'] = $project['revision'];
    }

Thus

projects[foo][download][branch] = "7.x-2.x"
projects[foo][revision] = "1234567"

always checks out the latest 2.x branch instead of the specified revision.

Since specifying a branch is advisable when using a specific revision (to avoid the 'unsupported release' messages the core update module throws), I'd like to remove the revision shortcut entirely in Drush 6, and fix the logic in 5.x.