absorb: import extension from Facebook's hg-experimental
absorb is a wicked-fast command to use blame information to
automatically amend edits to the correct draft revision. Originally
written by Jun Wu, this import is hgext3rd/absorb/__init__.py with:
* the `testedwith` value changed
* the linelog import updated
* some missing configitems registered
* some imports reordered per check-code.py
* some missing __future__ imports added per check-code.py
Differential Revision: https://phab.mercurial-scm.org/D3991