Drush makefiles should be able to flag individual projects to keep as working copies

Currently drush make supports a --working-copy option. This instructs any version control checkouts to maintain the checkouts as a working copy which can be independently managed (Updated etc.). However, this is an "all or nothing" approach.

It would be helpful if you could specify particular items to maintain working copies of without having to have it on for everything (Our example is we're basing the core install on a git checkout of pressflow which we *don't* want to keep the whole clone of, but some locally developed modules which we *do* want to keep as working copies).

The attached patch means that makefiles can provide a [download][working-copy] flag to specify that a project should maintain a working copy of that item, e.g.

projects[foo][type] = "module"
projects[foo][download][type] = "git"
projects[foo][download][url] = "ssh://git@gitrepo.example.com/foo.git"
projects[foo][download][working-copy] = "true"

Running such a makefile through drush make *without* the --working-copy flag means that any projects that don't specify the [working-copy] flag will be non-working copies, but projects that do specify it will be kept as working copies.