simplemerge: add optional context parameters to simplemerge Rename the existing parameters for clarity. These will, in subsequent patches, allow callers to redirect reads (of the three sides of the merge) and writes (of the result) to the given contexts, instead of using the filesystem. While in most cases, the writes will go to a workingfilectx, this opens the door for it to be a memfilectx in the case of an in-memory merge. Repo will be necessary in a subsequent comit. Differential Revision: https://phab.mercurial-scm.org/D372