t4207: test coloring of grafted decorations

t4207: test coloring of grafted decorations

The color.decorate.<slot> config option added the 'grafted' slot in
09c4ba410b0f (log-tree: allow to customize 'grafted' color, 2018-05-26)
but included no tests for this behavior. When modifying some logic
around decorations, this ref namespace was ignored and could have been
lost as a default namespace for 'git log' decorations by default.

Add two tests to t4207 that check that the replaced objects are
correctly decorated. Use "black" as the color since it is distinct from
the other colors already in the test. The first test uses regular
replace-objects while the second creates a commit graft.

Be sure to test both modes with GIT_REPLACE_REF_BASE unset and set to an
alternative base.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>