rebase: directly use '_destrebase' There is little value in using the revset instead of the function.

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.