Support fetch-jobs: 0 to use as many jobs as there are processors

The default is now fetch-jobs: -1, which is the new value for
reverting to normal git behavior (provide no --jobs argument).

__test__/input-helper.test.ts

@@ -75,7 +75,7 @@ describe('input-helper tests', () => {
     expect(settings.commit).toBeTruthy()
     expect(settings.commit).toBe('1234567890123456789012345678901234567890')
     expect(settings.fetchDepth).toBe(1)
-    expect(settings.fetchJobs).toBe(0)
+    expect(settings.fetchJobs).toBe(-1)
     expect(settings.lfs).toBe(false)
     expect(settings.ref).toBe('refs/heads/some-ref')
     expect(settings.repositoryName).toBe('some-repo')