git add --intent-to-add: do not let an empty blob be committed by accident

Writing a tree out of an index with an "intent to add" entry is blocked.
This implies that you cannot "git commit" from such a state; however you
can still do "git commit -a" or "git commit $that_path".

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

builtin-commit.c

@@ -639,7 +639,7 @@ static int prepare_to_commit(const char *index_file, const char *prefix)
 		active_cache_tree = cache_tree();
 	if (cache_tree_update(active_cache_tree,
 			      active_cache, active_nr, 0, 0) < 0) {
-		error("Error building trees; the index is unmerged?");
+		error("Error building trees");
 		return 0;
 	}