simplemerge: make `localorother` a "mode" instead of a separate thing `simplemerge()` takes a `mode` argument, which can be "union", "merge" or "mergediff", and a `localorother` argument, which can be `None`, "local", or "other". The two options are not at all orthogonal -- most combinations don't make sense. Also, at least "union", "local", and "other" are very closely related. Therefore, it makes sense to combine them into one. It probably makes sense to split the `mode` argument into `resolve` and `marker_style`, where the former can be `None`, "union", "local", or "other", and the latter can be "merge", "merge3", "mergediff", or "minimize". This is a good step in that direction whether or not we end up doing that. Differential Revision: https://phab.mercurial-scm.org/D11887