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
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}