Fix Git's config set with values containing white spaces.

The Salt function 'git.config_set' and the state 'git.config' (which relies
on the former) don't escape the value to be set in the configuration file
passed by Salt. For example, this doesn't work:

  mylocalrepo:
    git.config:
      - name: user.name
      - value: "Jonathan Ballet"
      - repo: file://my/path/to/repo

This set the 'user.name' configuration value to "Jonathan" only.
(Additionally, the state discovers that the value set is not the one
specified, and keeps trying to set the right value, which ends up with lot
of "name = Jonathan" into the "[user]" section of Git's config file.)

This commit fixes the problem and adds some unit tests to verify it's
working properly.