Executable flagging patch

[Gustavo Niemeyer]

> We might also want to have ChangeExecuteBit be aware of what rw bits are
> set. Because you may not want to enable X for a file which does not have
> rw for other.

We're not storing information about rw, so anything regarding these
is better put as non defined behavior.

> newpermissions = oldpermissions | (x_bit & oldmask)

Doing something similar in MergeTree.readonly_path would be nice
indeed.

-- 
Gustavo Niemeyer
http://niemeyer.net