whitespace: correct bit assignment comments

whitespace: correct bit assignment comments

A comment in diff.c claimed that bits up to 12th (counting from 0th)
are whitespace rules, and 13th thru 15th are for new/old/context,
but it turns out it was miscounting.  Correct them, and clarify
where the whitespace rule bits come from in the comment.  Extend bit
assignment comments to cover bits used for color-moved, which
weren't described.

Also update the way these bit constants are defined to use (1 << N)
notation, instead of octal constants, as it tends to make it easier
to notice a breakage like this.

Sprinkle a few blank lines between logically distinct groups of CPP
macro definitions to make them easier to read.

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