add nanoseconds test

__tests__/__fixtures__/test-GitIndex-timestamp/nano.txt

@@ -0,0 +1 @@
+validate nanoseconds

__tests__/test-GitIndex.js

@@ -2,7 +2,13 @@
 
 const path = require('path')
 
-const { GitIndex, GitIndexManager } = require('isomorphic-git/internal-apis')
+const {
+  GitIndex,
+  GitIndexManager,
+  FileSystem,
+} = require('isomorphic-git/internal-apis')
+
+const { normalizeStats } = require('../src/utils/normalizeStats.js')
 
 const { makeFixture } = require('./__helpers__/FixtureFS.js')
 
@@ -128,4 +134,23 @@ describe('GitIndex', () => {
     expect(ctimeSeconds).not.toBeNull()
     expect(ctimeNanoseconds).not.toBeNull()
   })
+
+  it('ensure nanoseconds of files are equal with index', async () => {
+    const _fs = Object.assign({}, require('fs'))
+    const fs = new FileSystem(_fs)
+
+    const buffer = await fs.read(path.join('.git/index'))
+    const index = await GitIndex.from(buffer)
+    const file = '__tests__/__fixtures__/test-GitIndex-timestamp/nano.txt'
+
+    const stats = normalizeStats(await fs.lstat(file))
+    expect(index.entriesMap.get(file).ctimeSeconds).toEqual(stats.ctimeSeconds)
+    expect(index.entriesMap.get(file).ctimeNanoseconds).toEqual(
+      stats.ctimeNanoseconds
+    )
+    expect(index.entriesMap.get(file).mtimeSeconds).toEqual(stats.mtimeSeconds)
+    expect(index.entriesMap.get(file).mtimeNanoseconds).toEqual(
+      stats.mtimeNanoseconds
+    )
+  })
 })