switch from native.git_repository to external git_repository

[excavador]

switch from native.git_repository to external git_repository from '@bazel_tools//tools/build_defs/repo:git.bzl' (#140)

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and have the pull request author add another comment and the bot will run again. If the bot doesn't comment, it means it doesn't think anything has changed.

[googlebot]

CLAs look good, thanks!

[erain]

Approved.

For now this is good but eventually we may want to switch all git_repository to http_archive since it is recommended.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: erain, excavador

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [erain]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[excavador]

So, can we merge this PR?

[erain]

/ok-to-test