Make --stdin option to "log" family read also pathspecs

Make --stdin option to "log" family read also pathspecs

Similar to the command line arguments, after giving zero or more revs, you can
feed a line "--" and then feed pathspecs one at a time.

With this

	(
		echo ^maint
		echo --
		echo Documentation
	) | git log --stat --oneline --stdin master -- t

lists commits that touch Documentation/ or t/ between maint and master.

Signed-off-by: Junio C Hamano <gitster@pobox.com>