filemerge: only raise InMemoryMergeConflictsError when running _xmerge...
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
