Remove `--pull` option from `bst shell`, and pull dependencies by default

Summary of changes:

  * _frontend/cli.py: Remove the `--pull` option

  * _stream.py: Remove `pull` option from `Stream.shell()`, and created
    a new method `_pull_missing_artifacts()` which is also used by
    Stream.checkout()

  * tests/integration/shell.py,shellbuildtrees.py: Updated test expectations

This fixes #685
4 files changed