export: don't prefetch *all* files in manifest
hg export only shows changed files, not all files, but we still
prefetched all files in cmdutil.export(). The same is true for the
other commands calling cmdutil.exportfile(). That meant that `hg
export` with remotefilelog (or lfs, I assume) could take much longer
than expected because it would download all the files in the repo.
Differential Revision: https://phab.mercurial-scm.org/D6532