streamclone: don't support stream clone unless repo feature present...
streamclone: don't support stream clone unless repo feature present
This change means custom repository types must opt in to enabling
stream clone. This seems reasonable, as stream clones are a very
low-level feature that has historically assumed the use of revlogs
and the layout of .hg/ that they entail.
Differential Revision:
https://phab.mercurial-scm.org/D4853