Investigate moving forward-porting to ORT merges and zdiff3 conflicts

[xmo-odoo]

Currently the mergebot does not configure the merge strategy for cherrypicking, and uses merge.conflictstyle=diff3 for markers.

• cherrypicking might be more reliable with the "ort" strategy, although maybe not as that supposedly is the default when "pulling or merging one branch" so maybe it's already in use

• zdiff3 can be slightly cleaner than diff3 visually (and less confusing) because it

  removes matching lines on the two sides from the conflict region when those matching lines appear near either the beginning or end of a conflict region.

  These are lines which don't appear in the conflict zone of a merge (diff2), because diff2 already moves them out of the conflict markers even though they're technically part of the conflict area.