exchange: teach pull about requested stream clones
An upcoming patch will move the invocation of stream cloning logic to
the normal pull code path (from localrepository.clone). In preparation
for this, we teach pull() and pulloperation about whether a streaming
clone is requested.
The return logic in localrepository.clone() has been reformatted
slightly because of line length issues.
streamclone: add explicit check for empty local repo
Stream clone doesn't work with non-empty local repositories. In upcoming
patches, we'll move stream cloning to the regular pull code path. Add an
explicit check on the repository being empty to prevent streaming clones
to non-empty repos.