rebase: extra default destination in its own function This makes it much simple to wrap for other extension.

revset: rename and test '_destmerge' We make the name consistent with the one used by '_destupdate' and we ensure the code is run by testing it (abort is expected and merge would).

merge: directly get destination from destutil There is no real value in using the revset over the function. The revset have no remaining users and will be taken care of in a later changesets.

destutil: move default merge destination into a function Function in destutil are much simpler to wrap and more flexible than revset. This also help consistency as 'destupdate' live here and cannot become a pure revset anyway.

revset: reintroduce and experimental revset for update destination The revset is not ready for prime time yet. However it is useful to have some version of it exposed to help candidate users to play with it and provide feedback on what we should aim at. We add a small test to make sure the code runs.

changegroup: move manifest unpacking into its own method The upcoming cg3 will need different logic for unpacking manifests.

changegroup: move manifest packing into a separate function A future change will introduce a new function on a cg3packer that can pack treemanifests as well as flatmanifests.