add diffstat section on `git log` and `git diff`.

log and diff related functions will display *diffstat* if `magit-show-diffstat` is non-nil.

On *diffstat* entry, `e` will launch `magit-ediff`.  However, the window configuration will not be restored properly yet.
fix a bug that can't handle abbreviated filename in diffstat

- better washing mechanism for diffstat sections
Merge remote-tracking branch 'refs/remotes/github/master'
Merge remote-tracking branch 'refs/remotes/github/master'

Conflicts:
	magit.el
Merge commit '3ac696bf59f960edb1d6578467e337f591c0f10c' into cinsk/master