diff: add experimental support for "merge diffs" The way this works is it re-runs the merge and "stores" conflicts, and then diffs against the conflicted result. In a normal merge, you should only see diffs against conflicted regions or in cases where there was a semantic conflict but not a textual one. This makes it easier to detect "evil merges" that contain substantial new work embedded in the merge commit. Differential Revision: https://phab.mercurial-scm.org/D8504