Improve terminal pager performance

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.

先定位終端分頁器，以及執行 git2cpp 命令並收集其輸出的進入點。調查輸出緩衝目前如何延遲顯示，並找出一種不需要獨立分頁器程序且與 cockle 相容的方法；當 git2cpp log 等大型命令能夠逐步顯示時，即表示完成。