Allow git to merge pbxproj files

Summary:
Follow on in https://github.com/facebook/react-native/pull/10864.

**Motivation**

ncuillery Is has a lot of experience with upgrading RN projects (see his [talk](http://www.slideshare.net/ncuillery/introducing-the-new-reactnative-upgrade)) and is rewriting 'react-native upgrade' to use git.

He tells me that we actually want git to merge changes in the .pbxproj file. In his words: "Making the project.pbxproj invisible for "git diff" means that react-native-git-upgrade will never be able to upgrade the project.pbxproj."

Note the git docs explicitly recommend not to use git to merge .pbxproj files: https://git-scm.com/book/en/v2/Customizing-Git-Git-Attributes#Binary-Files

ncuillery do we have an alternative? Some ideas:

1. Use a 3rd-party tool in 'react-native upgrade' to merge the .pbxproj file?
2. We could always re-generate and overwrite the .pbxproj file (never merge) and then run 'react-native link' to update native 3rd-party dependencies in the Xcode project?
3. Last resort: stay away
Closes https://github.com/facebook/react-native/pull/11047

Differential Revision: D4220521

fbshipit-source-id: 823c735856b519be114aa4349ca1392910f00445

local-cli/generator/copyProjectTemplateAndReplace.js

@@ -77,6 +77,7 @@ function dotFilePath(path) {
   if (!path) return path;
   return path
     .replace('_gitignore', '.gitignore')
+    .replace('_gitattributes', '.gitattributes')
     .replace('_babelrc', '.babelrc')
     .replace('_flowconfig', '.flowconfig')
     .replace('_buckconfig', '.buckconfig')

local-cli/templates/HelloWorld/.gitattributes

@@ -1 +0,0 @@
-*.pbxproj binary

local-cli/templates/HelloWorld/_gitattributes

@@ -0,0 +1 @@
+*.pbxproj -text