Use platform independent path separators in dedup

`npm dedupe` fails silently on windows due to incorrect
path splitting and joining without these changes.

lib/dedupe.js

@@ -130,15 +130,16 @@ function dedupe_ (dir, filter, unavoidable, dryrun, silent, cb) {
         // a=/path/to/node_modules/foo/node_modules/bar
         // b=/path/to/node_modules/elk/node_modules/bar
         // ==/path/to/node_modules/bar
-        a = a.split(/\/node_modules\//)
-        b = b.split(/\/node_modules\//)
+        var nmReg = new RegExp("\\" + path.sep + "node_modules\\" + path.sep)
+        a = a.split(nmReg)
+        b = b.split(nmReg)
         var name = a.pop()
         b.pop()
         // find the longest chain that both A and B share.
         // then push the name back on it, and join by /node_modules/
         var res = []
         for (var i = 0, al = a.length, bl = b.length; i < al && i < bl && a[i] === b[i]; i++);
-        return a.slice(0, i).concat(name).join("/node_modules/")
+        return a.slice(0, i).concat(name).join(path.sep + "node_modules" + path.sep)
       }) : undefined
 
       return [item[0], { item: item
@@ -192,9 +193,10 @@ function installAndRetest (set, filter, dir, unavoidable, silent, cb) {
       // where is /path/to/node_modules/foo/node_modules/bar
       // for package "bar", but we need it to be just
       // /path/to/node_modules/foo
-      where = where.split(/\/node_modules\//)
+      var nmReg = new RegExp("\\" + path.sep + "node_modules\\" + path.sep)
+      where = where.split(nmReg)
       where.pop()
-      where = where.join("/node_modules/")
+      where = where.join(path.sep + "node_modules" + path.sep)
       remove.push.apply(remove, others)
 
       return npm.commands.install(where, what, cb)

test/tap/dedupe.js

@@ -0,0 +1,29 @@
+var test = require("tap").test
+  , fs = require("fs")
+  , path = require("path")
+  , existsSync = fs.existsSync || path.existsSync
+  , npm = require("../../")
+  , rimraf = require("rimraf")
+
+test("dedupe finds the common module and moves it up one level", function (t) {
+  t.plan(1)
+
+  setup(function () {
+    npm.install(".", function (err) {
+      if (err) return t.fail(err)
+      npm.dedupe(function(err) {
+        if (err) return t.fail(err)
+        t.ok(existsSync(path.join(__dirname, "dedupe", "node_modules", "minimist")))
+      })
+    })
+  })
+})
+
+function setup (cb) {
+  process.chdir(path.join(__dirname, "dedupe"))
+  npm.load(function () {
+    rimraf.sync(path.join(__dirname, "dedupe", "node_modules"))
+    fs.mkdirSync(path.join(__dirname, "dedupe", "node_modules"))
+    cb()
+  })
+}

test/tap/dedupe/package.json

@@ -0,0 +1,9 @@
+{
+  "author": "Dedupe tester",
+  "name": "dedupe",
+  "version": "0.0.0",
+  "dependencies": {
+    "optimist": "0.6.0",
+    "clean": "2.1.6"
+  }
+}