Add test ensuring git repo matches manifest.
Add test ensuring git repo matches manifest.

This test guards against an issue encountered when the
initial gsutil 4.20 tarball included .travis.yml but that
file was not added to the manifest, causing installs of 4.20
to fail when running 'gsutil update' unless .travis.yml was
removed.

fishjord/gsutil,GoogleCloudPlatform/gsutil,BrandonY/gsutil,GoogleCloudPlatform/gsutil