shelve: use `targetphase` while unbundling This removes the last manual phase movement in shelve.

changegroup: add a `targetphase` argument to `addchangegroup` This argument controls the phase used for the added changesets. This can be useful to unbundle in "secret" phase as required by shelve. This change aims at helping high-level code get rid of manual phase movement. An important milestone for having phases part of the transaction.

shelve: do not retract phase boundary by hand We rely on the internal mechanism to commit the changeset in the right state. This is similar to what the mq extension is doing. This is an important change as we plan to move phase movement with the transaction. Avoiding phase movement from high level code will avoid them the burden of transaction handling. It is also important to limit the need for transaction handling as this limits the odds of people messing up. Most common expected mess-up is to use a different transaction for changesets creation and phase adjustment.