destupdate: move the check related to the "clean" logic in the function
We want this function to exactly predict the behavior for update. Moreover, we
would like to remove all high level behavior logic out of the merge module so
this is a step forward.
Now that the 'destupdate' function both compute and validate the destination, we
can directly use it at the command level, ensuring that the 'hg update' command
never call 'merge.update' without a defined destination. This is a first (but
significant) step toward having 'merge.update' always feed with a properly
validated destination and free of high level logic.