obsolete: add an explicit '_succs.copy()' method Mimic the standard API for copying in the _succs class, it makes the code slightly cleaner and will be needed later for copying markers at the same time than copying the list content.

obsolete: introduce a _succs class It will be useful later when we will be adding markers to _succs in order to represent a successorset with the list of markers from the root to each successors sets. This information will be needed for the obsfate template I will introduce. Makes it a subclass of list so all callers will continue to work.

simplemerge: simplify code now that we always write to a context There's no need for an `out` abstraction between files and contexts anymore. Differential Revision: https://phab.mercurial-scm.org/D383

simplemerge: make context parameters non-optional Also update the function docstring. Differential Revision: https://phab.mercurial-scm.org/D382

simplemerge: stop accepting, and passing, file parameters Differential Revision: https://phab.mercurial-scm.org/D381

simplemerge: stop reading from, and writing to, files We now use contexts first for everything and also pass them everywhere. Differential Revision: https://phab.mercurial-scm.org/D380

simplemerge: use context paths for default labels instead of file paths This is the last place we used the filepath arguments without first using the context version. Differential Revision: https://phab.mercurial-scm.org/D379

contrib: make simplemerge script pass context-like objects `simplemerge()` will soon require context-like objects to work. Create a simple context-like object that wraps the requested files and can be passed to the new API. Differential Revision: https://phab.mercurial-scm.org/D378