This is to help user prevent accidentally merge errors
and messing file layouts.
Its easy to turn whitespace ignore on for revieving
changes and forgot to turn it of before merging. Result
is often messed up file layout when indentation changes
are not merged.
This patch checks whitespace ignore setting before user
merges and shows informative warning message. Message
contains "Do not ask again" checkbox so this is only
one-time warning if user so wants.