Revert "unpack_trees(): skip trees that are the same in all input"

This reverts commit 83c90314aa, which
seems to have broken merge to report conflicts when there should be
none.

unpack-trees.c

@@ -427,10 +427,6 @@ static int switch_cache_bottom(struct traverse_info *info)
 	return ret;
 }
 
-static int fast_forward_merge(int n, unsigned long dirmask,
-			      struct name_entry *names,
-			      struct traverse_info *info);
-
 static int traverse_trees_recursive(int n, unsigned long dirmask,
 				    unsigned long df_conflicts,
 				    struct name_entry *names,
@@ -442,11 +438,6 @@ static int traverse_trees_recursive(int n, unsigned long dirmask,
 	struct traverse_info newinfo;
 	struct name_entry *p;
 
-	if (!df_conflicts) {
-		int status = fast_forward_merge(n, dirmask, names, info);
-		if (status)
-			return status;
-	}
 	p = names;
 	while (!p->mode)
 		p++;
@@ -694,53 +685,6 @@ static struct cache_entry *find_cache_entry(struct traverse_info *info,
 		return NULL;
 }
 
-static int fast_forward_merge(int n, unsigned long dirmask,
-			      struct name_entry *names,
-			      struct traverse_info *info)
-{
-	int i;
-	struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
-	struct unpack_trees_options *o = info->data;
-
-	/* merging two or more trees with an identical subdirectory? */
-	if ((n < 2) || ((1UL << n) - 1) != dirmask ||
-	    !o->merge || o->reset || o->initial_checkout)
-		return 0;
-	for (i = 1; i < n; i++)
-		if (hashcmp(names[i-1].sha1, names[i].sha1))
-			return 0;
-
-	/*
-	 * Instead of descending into the directory, keep the contents
-	 * of the current index.
-	 */
-	while (1) {
-		struct cache_entry *ce;
-		ce = next_cache_entry(o);
-		if (!ce)
-			break;
-		/* Is the entry still in that directory? */
-		if (do_compare_entry(ce, info, names))
-			break;
-		/*
-		 * Note: we do not just run unpack_index_entry() here,
-		 * as the callback may want to compare what is in the
-		 * index with what are from the HEAD and the other tree
-		 * and reject the merge.  We pretend that ancestors, the
-		 * HEAD and the other tree all have the same contents as
-		 * the current index, which is a lie, but it works.
-		 */
-		for (i = 0; i < n + 1; i++)
-			src[i] = ce;
-		mark_ce_used(ce, o);
-		if (call_unpack_fn(src, o) < 0)
-			return unpack_failed(o, NULL);
-		if (ce_stage(ce))
-			mark_ce_used_same_name(ce, o);
-	}
-	return dirmask;
-}
-
 static void debug_path(struct traverse_info *info)
 {
 	if (info->prev) {