[PATCH 0 of 2] log: add support for log --diffstat

[Yuya Nishihara]

It adds --diffstat option to log-like commands, like `hg log --diffstat'.

`hg log --diffstat' shows diffstat in place of patch output,
which means --diffstat supersedes --patch option.

I'm not sure about the option name. It can be 'log --stat'.