Do not overwrite the loaded tarball image

[davidcassany]

This commit makes sure that if the provided argument is a local
tarfile, the loaded image into the img varible does not get
overwritten by trying to fetch something from a remote registry.

From what I understand the img variable initialized in

container-diff/cmd/root.go

Line 132 in 0ffd08a

img, err = tarball.ImageFromPath(imageName, nil)

is always ignored by assuming the given argument is a remote reference.

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[davidcassany]

I signed it!

[davidcassany]

I signed it!

Hopefully with correct settings this time

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

memo Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.
What to do if you already signed the CLA
Individual signers

It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
The email used to register you as an authorized contributor must also be attached to your GitHub account.

[googlebot]

CLAs look good, thanks!

[nkubala]

Thanks for the fix!