cli.py: add a hint about '--' to 'bst shell' help

See merge request !1078

With out this, empty directories in the refs/heads directory just
grow unconditionally.

Enhance the test which checks removal of the extract directories
with an additional check that the ref directories are cleaned up
when removing artifacts.

When used in a child process in BuildStream, this should be
used instead of tempfile.NamedTemporaryFile() directly, otherwise
we fail to cleanup the file on SIGTERM.

The direct usage of tempfile.TemporaryDirectory() and
tempfile.NamedTemporaryFile() here causes leakage of any temporary
data when the process operating on temporary data is terminated
with SIGTERM.

Using the utilities ensures that trash is not left behind in
~/.cache/buildstream/artifacts/tmp when the user terminates
BuildStream with ^C.

CASCache cleanup improvements

See merge request !1087

Tristan Maat authored Jan 18, 2019 and Tristan Van Berkom committed Jan 18, 2019

`remove_artifact_from_cache` used a hard-coded path to remove
artifacts, which wasn't sufficient for integration tests.

Tristan Maat authored Jan 02, 2019 and Tristan Van Berkom committed Jan 18, 2019

This happened when bst is invoked with --message-lines 0 or
--error-lines 0, and was arguably a little too verbose (the user
explicitly asked us not to show them any lines, after all).

Fixes #779