tests/sitecustomize.py

patch: enable diff.tab markup for the color extension

The following patch splits up changed lines along tabs (using
re.findall), and gives them a "diff.tab" label. This can be used by
the color extension for colorising tabs, like it does right now with
trailing whitespace.

I also provide corresponding tests.

try:
    import coverage
    getattr(coverage, 'process_startup', lambda: None)()
except ImportError:
    pass