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
# See http://EditorConfig.org for the specification
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