merge: perform case-collision checking on final set of actions When there are multiple common ancestors, we should check for case collisions only on the resulting actions after bid merge has run. To do this, move the code until after bid merge. Move it past _resolvetrivial() too, since that might update actions. If the remote changed a file and then reverted the change, while the local side deleted the file and created a new file with a name that case-folds like the old file, we should fail before this patch but not after. Although the changes to the actions caused by _forgetremoved() should have no effect on case collisions, move it after that, too, so the next person reading the code won't have to think about it. Moving it past these blocks of code takes it to the end of calculateupdates(), so let's even move it outside of the method, so we also check collisions in actions produced by extensions overriding the method.