The local filesystem glob in `AssetSync::Storage#get_local_files` uses
fuzzy matching when config.prefix is present. This can present a problem
in some cases, as it doesn't allow for distinguishing between (e.g.) a
folder called `assets/` and another folder called `assets-temp/`. A
situation could arise where the latter folder has thousands/millions of
files and we mistakenly publish local-only files, or worse we could
clobber another directory in the bucket managed in a completely
different context.

This change allows developers to be more specific in their
`config.prefix` by using a trailing slash for their folder name.