global enhancements

- create context object and remove github one
- more tests and improve mocks

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

__tests__/git.test.ts

@@ -1,5 +1,6 @@
 import {beforeEach, describe, expect, it, jest} from '@jest/globals';
-import * as git from '../src/git';
+
+import {Git} from '../src/git';
 
 beforeEach(() => {
   jest.clearAllMocks();
@@ -8,7 +9,7 @@ beforeEach(() => {
 describe('git', () => {
   it('returns git remote ref', async () => {
     try {
-      expect(await git.getRemoteSha('https://github.com/docker/buildx.git', 'refs/pull/648/head')).toEqual('f11797113e5a9b86bd976329c5dbb8a8bfdfadfa');
+      expect(await Git.getRemoteSha('https://github.com/docker/buildx.git', 'refs/pull/648/head')).toEqual('f11797113e5a9b86bd976329c5dbb8a8bfdfadfa');
     } catch (e) {
       // eslint-disable-next-line jest/no-conditional-expect
       expect(e).toEqual(null);