Git 2.11.0 breaks the git clone approach in git_build_app_repo() in
25ab004c53
git worktree is a much simpler approach but only works for git >= 2.5
so it is not available in Ubuntu 14.04.
Fixes#2512.
This trigger allows you to modify the contents of an application *after* it has been extracted from git/tarball but *before* the image source type is detected. For example, you may wish to autogenerate a Dockerfile based on some other application contents before detecting what type of build process - herokuish or docker file - to use.
