Change data_dir argument in zoekt-dynamic-indexserver to repo_dir (#544)
This previously was only being used for storing thre repos. Since bare
repos take up much more disk space that the indexes and have different
IO requirements they need to be on a separate disk. We may later
introduce a more generic data_dir which can then set default values for
repo_dir and index_dir but for now it's simpler to make these
2 options explicit.