Change detection using git "three dot" diff (#35)

* Rework change detection via `git diff`

Previous implementation performed simple diff between two versions. New implementation fetches on demand more commits to have the merge base between two branches. Now it will detect only changes introduced by branch that was pushed, instead of mixing with changes introduced meanwhile on the base branch.

__tests__/git.test.ts

@@ -1,18 +1,11 @@
 import * as git from '../src/git'
-import {ExecOptions} from '@actions/exec'
 import {ChangeStatus} from '../src/file'
 
-describe('parsing of the git diff-index command', () => {
-  test('getChangedFiles returns files with correct change status', async () => {
-    const files = await git.getChangedFiles(git.FETCH_HEAD, (cmd, args, opts) => {
-      const stdout = opts?.listeners?.stdout
-      if (stdout) {
-        stdout(Buffer.from('A\u0000LICENSE\u0000'))
-        stdout(Buffer.from('M\u0000src/index.ts\u0000'))
-        stdout(Buffer.from('D\u0000src/main.ts\u0000'))
-      }
-      return Promise.resolve(0)
-    })
+describe('parsing output of the git diff command', () => {
+  test('parseGitDiffOutput returns files with correct change status', async () => {
+    const files = git.parseGitDiffOutput(
+      'A\u0000LICENSE\u0000' + 'M\u0000src/index.ts\u0000' + 'D\u0000src/main.ts\u0000'
+    )
     expect(files.length).toBe(3)
     expect(files[0].filename).toBe('LICENSE')
     expect(files[0].status).toBe(ChangeStatus.Added)