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>

graph.c

@@ -225,7 +225,12 @@ struct git_graph *graph_init(struct rev_info *opt)
 	graph->num_columns = 0;
 	graph->num_new_columns = 0;
 	graph->mapping_size = 0;
-	graph->default_column_color = 0;
+	/*
+	 * Start the column color at the maximum value, since we'll
+	 * always increment it for the first commit we output.
+	 * This way we start at 0 for the first commit.
+	 */
+	graph->default_column_color = COLUMN_COLORS_MAX - 1;
 
 	/*
 	 * Allocate a reasonably large default number of columns
@@ -499,11 +504,14 @@ static void graph_update_columns(struct git_graph *graph)
 			     parent;
 			     parent = next_interesting_parent(graph, parent)) {
 				/*
-				 * If this is a merge increment the current
+				 * If this is a merge, or the start of a new
+				 * childless column, increment the current
 				 * color.
 				 */
-				if (graph->num_parents > 1)
+				if (graph->num_parents > 1 ||
+				    !is_commit_in_columns) {
 					graph_increment_column_color(graph);
+				}
 				graph_insert_into_new_columns(graph,
 							      parent->item,
 							      &mapping_idx);