Update test script, export getGitDiff, and add unit test for it in te…

…st.js.

package.json

@@ -8,7 +8,7 @@
     "gitai": "./bin/gitai.js"
   },
   "scripts": {
-    "test": "echo \"Error: no test specified\" && exit 1",
+    "test": "node --test",
     "lint": "standard --fix 'src/*.js'",
     "version": "git add -A",
     "prerelease": "npm run lint",

src/index.js

@@ -58,3 +58,5 @@ program
   })
 
 program.parse()
+
+module.exports = { getGitDiff }

test/test.js

@@ -0,0 +1,10 @@
+const assert = require('node:assert')
+const test = require('node:test')
+const { getGitDiff } = require('../src/index.js')
+
+test('getGitDiff', async (t) => {
+  await t.test('should return a string', () => {
+    const diff = getGitDiff()
+    assert.strictEqual(typeof diff, 'string')
+  })
+})