commands: support consuming stream clone bundles
For the same reasons that we don't produce stream clone bundles with `hg
bundle`, we don't support consuming stream clone bundles with `hg
unbundle`. We introduce a complementary debug command for applying
stream clone bundles. This command is mostly to facilitate testing.
Although it may be used to manually apply stream clone bundles until a
more formal mechanism is (possibly) adopted.
(binary:application/octet-stream)