context: add arbitraryfilectx, which can represent files outside the workdir
Move it from contrib/simplemerge so it can be re-used in the future.
Differential Revision: https://phab.mercurial-scm.org/D604
simplemerge: remove unused `repo` parameter
This is now no longer used or needed thanks to the `decodeddata()` context
function.
Differential Revision: https://phab.mercurial-scm.org/D602