store: lazily get file size on demand for the fncache case
We don't have the information in the first place, so we can avoid querying the
file system inconditionnaly for use case we don't needs it.
This change requires the StoreFile class to be passed a vfs to retrieve the
file_size if needed.
In the non-fncache case, we already have the information from file system
walking, so we keep it and use it.
# So GitLab doesn't think we're using tons of Perl
*.t -linguist-detectable