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
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