linelog: add a Python implementation of the linelog datastructure This datastructure was originally developed by Jun Wu at Facebook, inspired by SCCS weaves. It's useful as a cache for blame information, but also is the magic that makes `hg absorb` easy to implement. In service of importing the code to Mercurial, I wanted to actually /understand/ it, and once I did I decided to take a run at implementing it. The help/internals/linelog.txt document is the README from Jun Wu's implementaiton. It all applies to our linelog implementation. Differential Revision: https://phab.mercurial-scm.org/D3990