Resolve "fatal: not a git repository" error outside of git repositories

Summary:
CocoaPods will display a "fatal: not a git repository" when these podspecs are consumed within Facebook's internal Mercurial repository due to the reliance on `git` to obtain the current commit hash.

In these cases, the podspec is being consumed locally and the commit hash is unnecessary.

The error is removed by avoiding the use of `git` if the current working directory is not a git repository (or any of the parent directories).

Changelog:
[Internal] [iOS] - Remove CocoaPods error within Facebook's repository

Reviewed By: fkgozali

Differential Revision: D27750974

fbshipit-source-id: 99159611c580baf5526f116948c5ff60e1c02e5c