Optimization opportunity: only fetch necessary branches

[felipecrs]

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During the repository clone, Renovate will fetch all branches.

I believe this flow can be optimized: instead of fetching all branches, it can:

1. Fetch only the default branch (HEAD), <branchPrefix>/* and maybe also <oldBranchPrefix>/*
2. Based on the baseBranches configuration, fetch additional branches. For most cases, no additional branches will need to be fetched.

For very big repositories with several branches this can make a big difference.