Improve graph rendering in the default graphic mode

Using the same symbol for ordinary and boundary commits is misleading.
Keep 'o' for boundary commits only, it's similar to other graphic modes.

Use ACS_BULLET for ordinary commits, ACS_DIAMOND for merges and '@' for
the initial commit, which is similar to the symbols used in the UTF-8
graphic mode.

src/graph-v1.c

@@ -396,11 +396,11 @@ graph_symbol_to_chtype(const struct graph_symbol *symbol)
 		if (symbol->boundary)
 			graphics[1] = 'o';
 		else if (symbol->initial)
-			graphics[1] = 'I';
+			graphics[1] = '@';
 		else if (symbol->merge)
-			graphics[1] = 'M';
+			graphics[1] = ACS_DIAMOND;
 		else
-			graphics[1] = 'o'; //ACS_DIAMOND; //'*';
+			graphics[1] = ACS_BULLET;
 		return graphics;
 	}
 

src/graph-v2.c

@@ -1096,11 +1096,11 @@ graph_symbol_to_chtype(const struct graph_symbol *symbol)
 		if (symbol->boundary)
 			graphics[1] = 'o';
 		else if (symbol->initial)
-			graphics[1] = 'I';
+			graphics[1] = '@';
 		else if (symbol->merge)
-			graphics[1] = 'M';
+			graphics[1] = ACS_DIAMOND;
 		else
-			graphics[1] = 'o'; //ACS_DIAMOND; //'*';
+			graphics[1] = ACS_BULLET;
 		return graphics;
 
 	} else if (graph_symbol_cross_merge(symbol)) {