Properly handle cases with tmp git repos are corrupted

[kpdecker]

For some reason the tmp directory here appears to have been corrupted. In this case we should retry the operation with a clean clone.

[13:30:45] Starting 'version'...
fatal: Not a git repository (or any of the parent directories): .git

/Users/kpdecker/dev/walmart/generator-release/lib/git.js:486
              throw new Error('Failed executing ' + args + '\n  cwd: ' + proce
                    ^
Error: Failed executing git,pull,--ff-only
  cwd: /private/tmp/components_vendor-thorax

    at ChildProcess.<anonymous> (/Users/kpdecker/dev/walmart/generator-release/lib/git.js:486:21)
    at ChildProcess.emit (events.js:98:17)
    at Process.ChildProcess._handle.onexit (child_process.js:820:12)