Improve terminal pager performance #131

The terminal pager is quite simple, it waits for the git2cpp command to produce all of its output before it is displayed. This can be slow for e.g. a git2cpp log of a big repo. Commands in a real terminal would run the pager in a separate process and pipe the output from the command to it, so it fills progressively. We cannot run the pager as a separate process in cockle so we'll need to find some other way of doing this better.