Create ~/.ssh directory when using Git via SSH

Previously, if the `${HOME}/.ssh` directory did not already exist when
attempting to checkout a Git repository via SSH, the
`checkout edx-platform repo` and `checkout theme` tasks would fail.

This issue doesn't appear when checking out code as the `edxapp` user
(default: presumably this directory is created elsewhere), but will
surface if run as another user who lacks a `${HOME}/.ssh` directory.

The code now looks before it leaps.