cmds: disable external diff when generating diffstats

Selecting top-level items in the status widget can invoke an external
diff command when `diff.external` is defined.  Protect ourselves against
this by passing '--no-ext-diff' when invoking "git diff".

Closes #163

Reported-by: @jeff-dagenais via github.com
Signed-off-by: David Aguilar <davvid@gmail.com>