pipcl.py: various improvements.

* Added some venv_*() fns.
* build_extension(): added source_extra arg.
* run_if(): improved display of changed commands.
* git_get(): improved API.
* NewFiles: added `get_n()` method.
* Don't special-case graal builds when running under cibuildwheel.
* run_if(): fix handling of commands that differ only in white space.

Author: julian-smith-artifex-com