fix(git): strip GIT environs when running git

When running an npm command from within a git environment, such as
installing or testing during a git rebase or bisect, these environment
variables will be passed to the child process, causing it to
fetch/checkout/etc in the root project instead of doing what the user
intends.

Strip them out so that they are not passed to the child process.

Also, remove git environs from the test environment, so that spawning
git in a test to set up a dummy repo doesn't mess with the main
project's git repository.  This enables adding `exec npm test` in a `git
rebase -i` list to run tests between commits.

lib/utils/git.js

@@ -28,6 +28,16 @@ function execGit (args, options, cb) {
 
 function spawnGit (args, options) {
   log.info('git', args)
+  // If we're already in a git command (eg, running test as an exec
+  // line in an interactive rebase) then these environment variables
+  // will force git to operate on the current project, instead of
+  // checking out/fetching/etc. whatever the user actually intends.
+  options.env = options.env || Object.keys(process.env)
+    .filter(k => !/^GIT/.test(k))
+    .reduce((set, k) => {
+      set[k] = process.env[k]
+      return set
+    }, {})
   return spawn(git, prefixGitArgs().concat(args || []), options)
 }
 

test/common-tap.js

@@ -7,6 +7,12 @@ var readCmdShim = require('read-cmd-shim')
 var isWindows = require('../lib/utils/is-windows.js')
 var Bluebird = require('bluebird')
 
+// remove any git envs so that we don't mess with the main repo
+// when running git subprocesses in tests
+Object.keys(process.env).filter(k => /^GIT/.test(k)).forEach(
+  k => delete process.env[k]
+)
+
 // cheesy hackaround for test deps (read: nock) that rely on setImmediate
 if (!global.setImmediate || !require('timers').setImmediate) {
   require('timers').setImmediate = global.setImmediate = function () {