filemerge: add a hacktastic version of internal:merge3 for merge diffs
This is a version of merge3 that always reports success, so that
conflict markers get preserved without us having to implement conflict
storage for in-memory merge. Credit to martinvonz for the idea. The
only planned consumer of this "merge tool" is my upcoming merge-diffs
functionality, though I suspect it could be useful in other ways.
Differential Revision: https://phab.mercurial-scm.org/D8515
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