filemerge: only raise InMemoryMergeConflictsError when running _xmerge The old code here was overly broad and would raise in cases when we didn't end up calling `xmerge` and resolved using an internal tool (such as when `premerge=True`). Instead, let's swap out _xmerge if IMM is enabled and have the new tool raise when called, which is the behavior we want. Differential Revision: https://phab.mercurial-scm.org/D1739