write-tree: properly detect failure to write tree objects

Tomasz Fortuna reported that "git commit" does not error out properly when
it cannot write tree objects out.  "git write-tree" shares the same issue,
as the failure to notice the error is deep in the logic to write tree
objects out recursively.

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

cache-tree.c

@@ -341,8 +341,11 @@ static int update_one(struct cache_tree *it,
 
 	if (dryrun)
 		hash_sha1_file(buffer.buf, buffer.len, tree_type, it->sha1);
-	else
-		write_sha1_file(buffer.buf, buffer.len, tree_type, it->sha1);
+	else if (write_sha1_file(buffer.buf, buffer.len, tree_type, it->sha1)) {
+		strbuf_release(&buffer);
+		return -1;
+	}
+
 	strbuf_release(&buffer);
 	it->entry_count = i;
 #if DEBUG