add--interactive: compute the empty tree value

The interactive add script hard-codes the object ID of the empty tree.
To avoid any problems when changing hashes, compute this value when used
and cache it for any future uses.
Signed-off-by: Nbrian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-add--interactive.perl

@@ -205,8 +205,15 @@ sub list_untracked {
 	}
 }
 
-sub get_empty_tree {
-	return '4b825dc642cb6eb9a060e54bf8d69288fbee4904';
+{
+	my $empty_tree;
+	sub get_empty_tree {
+		return $empty_tree if defined $empty_tree;
+
+		$empty_tree = run_cmd_pipe(qw(git hash-object -t tree /dev/null));
+		chomp $empty_tree;
+		return $empty_tree;
+	}
 }
 
 sub get_diff_reference {