rebase: rename and test '_destrebase' We make the name consistent with the other similar revsets and make sure it has minimal tests.

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).