CodingGuidelines: spaces around C operators

CodingGuidelines: spaces around C operators

As we have operated with "write like how your surrounding code is
written" for too long, after a huge code drop from another project,
we'll end up being inconsistent before such an imported code is
cleaned up.  We have many uses of cast operator with a space before
its operand, mostly in the reftable code.

Spell the convention out before it spreads to other places.

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