graph API: use a new color when starting a brand new column

graph API: use a new color when starting a brand new column

Use a new color for commits that don't have any previously printed
children.  The following command demonstrates the changes:

  git log --graph --pretty=tformat:'%h %s%n' -7 481c7a6 18b0793

Now the two independent lines of development are displayed with
different colors, instead of both using the same color.

Signed-off-by: Adam Simpkins <simpkins@facebook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>