unpack-trees: add usage notices around df_conflict_entry

Avoid making users believe they need to initialize df_conflict_entry
to something (as happened with other output only fields before) with
a quick comment and a small sanity check.

Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

unpack-trees.c

@@ -1876,6 +1876,8 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 		BUG("o->internal.dir is for internal use only");
 	if (o->internal.pl)
 		BUG("o->internal.pl is for internal use only");
+	if (o->df_conflict_entry)
+		BUG("o->df_conflict_entry is an output only field");
 
 	trace_performance_enter();
 	trace2_region_enter("unpack_trees", "unpack_trees", the_repository);

unpack-trees.h

@@ -78,7 +78,7 @@ struct unpack_trees_options {
 
 	int head_idx;
 
-	struct cache_entry *df_conflict_entry;
+	struct cache_entry *df_conflict_entry; /* output only */
 	void *unpack_data;
 
 	struct index_state *dst_index;