Update Git.clone to set multiple config variables

[jcouball]

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Description

Currently, Git.clone only allows setting a single config variable via the :config option. This option is given as a string as follows:

Git.clone(uri, destination, { config: 'submodule.recurse=true' }

This PR allows an Array to be given for :config (in addition to still supporting the single String value):

g = Git.clone(
  uri, destination,
  :config => [
    'core.sshCommand=ssh -i /home/user/.ssh/id_rsa',
    'submodule.recurse=true'
  ]
)

See [git-clone --config](https://git-scm.com/docs/git-clone#Documentation/git-clone.txt--cltkeygtltvaluegt) for more details.