You seem to be on the right track. Here is what I use, which works and fixes correctly:
core = "6.x"
api = "2"
projects[drupal][type] = "core"
projects[drupal][version] = "6.28"
projects[drupal][patch][] = "http://drupalcode.org/project/openatrium.git/blob_plain/refs/heads/master:/robots_txt_rollback.patch"
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