graph API: fix extra space during pre_commit_line state

An extra space is being inserted between the "commit" column and all of
the successive edges.  Remove this space.  This regression was
introduced by 427fc5b.

Signed-off-by: Allan Caffee <allan.caffee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

graph.c

@@ -727,8 +727,8 @@ static void graph_output_pre_commit_line(struct git_graph *graph,
 		if (col->commit == graph->commit) {
 			seen_this = 1;
 			strbuf_write_column(sb, col, '|');
-			strbuf_addf(sb, " %*s", graph->expansion_row, "");
-			chars_written += 2 + graph->expansion_row;
+			strbuf_addf(sb, "%*s", graph->expansion_row, "");
+			chars_written += 1 + graph->expansion_row;
 		} else if (seen_this && (graph->expansion_row == 0)) {
 			/*
 			 * This is the first line of the pre-commit output.