subrepo: pull revisions on demand when archiving hg subrepos
Before, running 'hg archive -S' could result in
abort: unknown revision '
65903cebad86f1a84bd4f1134f62fa7dcb7a1c98'!
if a subrepo was missing completely or had missing changesets. Now,
the missing changesets will be pulled or cloned as appropriate.
This make Mercurial subrepos match Git subrepos which already took
care to fetch any missing commits before starting the archive.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.