Improve change detection for feature branches (#16)

* Detect changes against configured base branch

* Update README and action.yml

* Add job.outputs example

* Update CHANGELOG

__tests__/git.test.ts

@@ -0,0 +1,19 @@
+import * as git from '../src/git'
+
+describe('git utility function tests (those not invoking git)', () => {
+  test('Detects if ref references a tag', () => {
+    expect(git.isTagRef('refs/tags/v1.0')).toBeTruthy()
+    expect(git.isTagRef('refs/heads/master')).toBeFalsy()
+    expect(git.isTagRef('master')).toBeFalsy()
+  })
+  test('Trims "refs/" from ref', () => {
+    expect(git.trimRefs('refs/heads/master')).toBe('heads/master')
+    expect(git.trimRefs('heads/master')).toBe('heads/master')
+    expect(git.trimRefs('master')).toBe('master')
+  })
+  test('Trims "refs/" and "heads/" from ref', () => {
+    expect(git.trimRefsHeads('refs/heads/master')).toBe('master')
+    expect(git.trimRefsHeads('heads/master')).toBe('master')
+    expect(git.trimRefsHeads('master')).toBe('master')
+  })
+})