[WIP] diff tools

[asmallteapot]

intent of this PR is to add support for using the xcodeproj command as a git difftool.

[CocoaPodsBarista]

	1 Warning
⚠️	Please include a CHANGELOG entry to credit yourself! You can find it at CHANGELOG.md.

Here's an example of your CHANGELOG entry:

* [WIP] diff tools  
  [asmallteapot](https://github.com/asmallteapot)
  [#issue_number](https://github.com/CocoaPods/CocoaPods/issues/issue_number)

note: There are two invisible spaces after the entry's text.

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[asmallteapot]

The first build failed because master is currently stuck on 1.5.9. Should I create a PR to bump master, or is it being held back intentionally?

[segiddins]

Looks like travis is failing to install bundler?

[dnkoutso]

rebase might fix it. if not I can assist.

[dnkoutso]

@asmallteapot is this change considered complete?

[asmallteapot]

remaining tasks for this PR

• Fix xcodeproj --help not showing the diff subcommand
• Add CHANGELOG.md entry

[segiddins]

I'm nervous that no tests broke due to this change. Additionally, does this change the command names used to access the commands? If so, that's a backwards-incompatible change