examples/diff:Add the --shortstat flag to examples/diff.c based on --numstat.

* My work is based on the work that was done to add --numstat.
    * mind the singular and plural forms of the output.
    * add a field of `shortstat` in `struct opts` and follow the things 'numstat' do.
* Remove the corresponding information from the PROJECTS.md


Regards,
He Sun