unpack_trees(): fix diff-index regression.

When skip_unmerged option is not given, unpack_trees() should not just
skip unmerged cache entries but keep them in the result for the caller to
sort them out.

For callers other than diff-index, the incoming index should never be
unmerged, but diff-index is a special case caller.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff-lib.c

@@ -641,6 +641,21 @@ static void do_oneway_diff(struct unpack_trees_options *o,
 	show_modified(revs, tree, idx, 1, cached, match_missing);
 }
 
+static inline void skip_same_name(struct cache_entry *ce, struct unpack_trees_options *o)
+{
+	int len = ce_namelen(ce);
+	const struct index_state *index = o->src_index;
+
+	while (o->pos < index->cache_nr) {
+		struct cache_entry *next = index->cache[o->pos];
+		if (len != ce_namelen(next))
+			break;
+		if (memcmp(ce->name, next->name, len))
+			break;
+		o->pos++;
+	}
+}
+
 /*
  * The unpack_trees() interface is designed for merging, so
  * the different source entries are designed primarily for
@@ -662,6 +677,9 @@ static int oneway_diff(struct cache_entry **src, struct unpack_trees_options *o)
 	struct cache_entry *tree = src[1];
 	struct rev_info *revs = o->unpack_data;
 
+	if (idx && ce_stage(idx))
+		skip_same_name(idx, o);
+
 	/*
 	 * Unpack-trees generates a DF/conflict entry if
 	 * there was a directory in the index and a tree

unpack-trees.c

@@ -116,7 +116,6 @@ static int unpack_index_entry(struct cache_entry *ce, struct unpack_trees_option
 			add_entry(o, ce, 0, 0);
 			return 0;
 		}
-		return 0;
 	}
 	return call_unpack_fn(src, o);
 }
@@ -286,7 +285,6 @@ static int unpack_callback(int n, unsigned long mask, unsigned long dirmask, str
 						add_entry(o, ce, 0, 0);
 						return mask;
 					}
-					continue;
 				}
 				src[0] = ce;
 			}