Auto merge of #99149 - ferrocene:pa-nightly-branch, r=Mark-Simulacrum

Configure nightly branch name in `stage0.json`

The beta version number detection code relies on git to know how many merge commits were made since we branched off, and in doing so hardcodes `master` as the default branch name. This works for rust-lang/rust, but is problematic for forks that use a different default branch name (in Ferrocene we use `main` instead).

This PR changes the code to instead load the default branch name from `src/stage0.json`. `bump-stage0` has also been updated to remove the need to update it every time a new field is added to `stage0.json`.

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