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

simplemerge: use `ctx.decoddeddata()` instead of `repo.wreaddata` This eliminates the need for the `repo` object. Differential Revision: https://phab.mercurial-scm.org/D435

context: add `decodeddata()` to basefilectx This will be used as an abstraction by simplemerge to get the data it used to read off the filesystem. Differential Revision: https://phab.mercurial-scm.org/D434