Automatic merge from submit-queue. 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>.

git: Use VolumeHost.GetExec() to execute stuff in volume plugins

This prepares volume plugins to run things in containers instead of running
them on the host.

**Special notes for your reviewer**:

* instead of `cmd.SetDir(<dir>); cmd.Exec("git <command>")`, we do `cmd.Exec("git -C <dir> <command>")` - mounter.Exec does not have SetDir()
* there are lot of changes in the tests because a different exec interface is used.

@kubernetes/sig-storage-pr-reviews 

```release-note
gitRepo volumes in pods require git 1.8.5 or later
```

/assign @rootfs