default download type to git when download revision key is set

since the drupal repositories are all in git now, it would be nice if the download type, if not set, would default to "git" in the case where a download revision is specified. that way, instead of this:

project[views][version] = 2.x-dev
project[views][download][type] =  "git"
project[views][download][revision] =  "a6s5d4f6a5s4"

you could do this:

project[views][version] = 2.x-dev
project[views][download][revision] =  "a6s5d4f6a5s4"

given that as part of http://drupal.org/node/779440, we'd like to encourage distro maintainers to use specific hashes in the case where they are using a development branch in their distro, this change will make that easier.

i might even suggest that we provide a convenience shortcut of specifying revision as a top level attribute, like so:

project[views][version] = 2.x-dev
project[views][revision] =  "a6s5d4f6a5s4"

and drush make would internally set all the appropriate download settings.