This will be used to prime the RepoSizes database, which will always
contain values that correpond to information in the git-annex branch, so
without anything from journal files.
Factored out overJournalFileContents which will later be used to
update Annex.reposizes to include information from journal files.
This will be partitcularly important to support private UUIDs which only
ever get to journal files and not to the branch.
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