git-count-objects: dc replacement

git-count-objects: dc replacement

Johannes suggested this earlier but I did not take it so
seriously because this command is not that important.  But this
probably matters on Cygwin which does not seem to come with
precompiled dc.  It is a mystery for me that anything that
mimics UNIX does not offer a dc, though.

I did the detection for the lack of dc command a bit differently
from the verison Johannes did.

Signed-off-by: Junio C Hamano <junkio@cox.net>