Helpers to translate between branch indexes and branch names

[wlandau]

Prework

• Read and agree to the code of conduct and contributing guidelines.
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• Format your code according to the tidyverse style guide.

Proposal

• tar_branch_index(): accept a character vector of branch names and return an integer vector of branch indexes as recorded in the metadata.
• tar_branch_names(): accept an integer vector of branch indexes and return a character vector of branch names as recorded in the metadata.