REGRESSION: Performance

[boyter]

Describe the bug

The latest release which includes the new git logic pulled from gocodewalker has made scc much slower than is used to be.

To Reproduce

Compare version before and after the change.

Expected behavior

Would be nice to have the speed back.

[boyter]

First tweak by modifying gocodewalker to walk in parallel and cache some calls. Still more work needed to close the gap, as on a 32 core machine scc counts the linux kernel in ~1 second while tokei is ~700ms.