Allow to check out existing git branch

Currently, if a git branch is specified, drush make tries to create a local branch automatically. For [branch] = "dev", this is e.g.
git checkout -b dev origin/dev

This fails if the branch already exists. This happens when the branch happens to be the branch automatically checked out upon clone (normally, this is "master", but that's just convention).

With the attached patch, it first tries to check out the branch directly, and if that fails, tries to check out origin/branchname (as now).