Allow users to specify the default branch name

While users usually use `master` as the main/default branch, there
are cases in which `master` may need to be the distribution branch
(e.g.: the GitHub User & Organization Pages¹ use the content from
the `master` branch, so users usually place the raw source code in
a different branch).

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¹ https://help.github.com/articles/user-organization-and-project-pages/#user--organization-pages

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