Automatic merge from submit-queue (batch tested with PRs 63445, 63820). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

core v1: deprecate the gitRepo volume type

gitRepo stopped accepting new features nearly 2 years ago https://github.com/kubernetes/kubernetes/issues/17676#issuecomment-228650586 and today this behavior can easily be achieved through an init container. The kubelet shelling out to git in the host namespace can also be a security issue on un-trusted repos, as was demonstrated by [CVE-2017-1000117](https://groups.google.com/forum/#!topic/kubernetes-announce/CTLXJ74cu8M). Our own documentation even alludes to this volume type being removed in the future:

> In the future, such volumes may be moved to an even more decoupled model, rather than extending the Kubernetes API for every such use case.

https://kubernetes.io/docs/concepts/storage/volumes/#gitrepo

Closes https://github.com/kubernetes/kubernetes/issues/60999

```release-note-action-required
The GitRepo volume type is deprecated. To provision a container with a git repo, mount an EmptyDir into an InitContainer that clones the repo using git, then mount the EmptyDir into the Pod's container.
```

/release-note-action-required

Instead of this:

```yaml
apiVersion: v1
kind: Pod
metadata:
  name: server
spec:
  containers:
  - image: nginx
    name: nginx
    volumeMounts:
    - mountPath: /mypath
      name: git-volume
  volumes:
  - name: git-volume
    gitRepo:
      repository: "git@somewhere:me/my-git-repository.git"
      revision: "22f1d8406d464b0c0874075539c1f2e96c253775"
```

Do this:

```yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: git-clone
data:
  git-clone.sh: |
    #!/bin/sh -e
    git clone $1 $3
    cd $3
    git reset --hard $2
---
apiVersion: v1
kind: Pod
metadata:
  name: server
spec:
  initContainers:
  - name: git-clone
    image: alpine/git # Any image with git will do
    command:
    - /usr/local/git/git-clone.sh
    args:
    - "https://somewhere/me/my-git-repository.git"
    - "22f1d8406d464b0c0874075539c1f2e96c253775"
    - "/mypath"
    volumeMounts:
    - name: git-clone
      mountPath: /usr/local/git
    - name: git-repo
      mountPath: /mypath
  containers:
  - image: nginx
    name: nginx
    volumeMounts:
    - mountPath: /mypath
      name: git-volume
  volumes:
  - name: git-volume
    emptyDir: {}
  - name: git-clone
    configMap:
      name: git-clone
      defaultMode: 0755
```