To create a new directory, you can simply create it in your local directory (i.e. cd app; mkdir log then commit and click on Heroku.
Whether /app top directory or independent of your directory structure, this is usually the name of the Heroku application that is installed as the top directory. Although, if you are in your git repository that was used for your Heroku application, you can find the root directory:
git rev-parse --show-toplevel
which is a git command that tells the top level directory of this git directory.
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