unpack-trees: preserve the index file version of original

Otherwise "git checkout $other_branch" (or even "git checkout HEAD")
would end up writing the index out in the default format.

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

unpack-trees.c

@@ -1020,6 +1020,7 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	o->result.initialized = 1;
 	o->result.timestamp.sec = o->src_index->timestamp.sec;
 	o->result.timestamp.nsec = o->src_index->timestamp.nsec;
+	o->result.version = o->src_index->version;
 	o->merge_size = len;
 	mark_all_ce_unused(o->src_index);