test(git_service): remove circular dependency and add tests

This is the first out of two MRs migrating the largest file in the codebase (git_service.js) to TypeScript.

This MR makes several changes:

• It breaks a circular dependency on status_bar.
• It improves our test suite by adding more tests.
• It refactors the current tests so that each test suite sets up its own mocked API endpoints (until this MR, all tests shared the same mock API setup which started getting too large).
• It migrates ApiError to TS.
• It simplified running integration tests from the VS Code.

Part of #220 (closed)