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.

首先定位终端分页器，以及执行 git2cpp 命令并收集其输出的入口点。调查输出缓冲目前如何延迟显示，并确定一种无需单独分页器进程且与 cockle 兼容的方法；当 git2cpp log 等大型命令能够逐步显示时，即表示完成。