The four callers of GetBaseBranch all ultimately want a single ref,
but the disambiguation prompt we're about to add needs to know when
multiple main branches are genuinely tied at the closest position so
it can ask the user. Change the function to return all min-ahead refs
in config order and rename to GetBaseBranchCandidates; each caller
now takes candidates[0] as a placeholder until the prompt wiring
lands.
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