feat: add support for git amend (#126)

* feat: add support for git amend

So as to allow users to maintain there "one commit" [style][eslint],
this PR adds the ability to specify whether or not to have the lockfile
update be amended into the previous commit.

[eslint]: https://eslint.org/docs/1.0.0/developer-guide/contributing#step-5-squash-your-commits

* test: remove duplicates

.gitignore

@@ -1,3 +1,4 @@
+coverage
 node_modules
 *.log
 # jest coverage output

lib/update-lockfile.js

@@ -1,7 +1,7 @@
 'use strict'
 
 const exec = require('child_process').execSync
-
+const _ = require('lodash')
 const semver = require('semver')
 
 const flags = {
@@ -47,15 +47,22 @@ module.exports = function updateLockfile (dependency, options) {
 
   const commitEmail = process.env.GK_LOCK_COMMIT_EMAIL ? process.env.GK_LOCK_COMMIT_EMAIL.trim() : 'support@greenkeeper.io'
   const commitName = process.env.GK_LOCK_COMMIT_NAME ? process.env.GK_LOCK_COMMIT_NAME.trim() : 'greenkeeperio-bot'
+  const shouldAmend = !_.includes([undefined, `0`, 'false', 'null', 'undefined'], process.env.GK_LOCK_COMMIT_AMEND)
 
   if (exec('git status --porcelain').toString() === '') return
 
-  // commit the updated lockfile
+  // stage the updated lockfile
   exec('git add npm-shrinkwrap.json 2>/dev/null || true')
   exec('git add package-lock.json 2>/dev/null || true')
   exec('git add yarn.lock 2>/dev/null || true')
+
   exec(`git config user.email "${commitEmail}"`)
   exec(`git config user.name "${commitName}"`)
-  const updateMessage = 'chore(package): update lockfile\n\nhttps://npm.im/greenkeeper-lockfile'
-  exec(`git commit -m "${updateMessage}"`)
+
+  if (shouldAmend) {
+    exec(`git commit --amend --author="${commitName} <${commitEmail}>" --no-edit`)
+  } else {
+    const updateMessage = 'chore(package): update lockfile\n\nhttps://npm.im/greenkeeper-lockfile'
+    exec(`git commit -m "${updateMessage}"`)
+  }
 }