merge: add a config to allow conflict-free merge of changes on adjacent lines This change adds a config to make it no longer a conflict to merge changes made on adjacent lines. The reason these changes are considered a conflict is that there's no region of text at the relevant position (sync region) that's kept unchanged by both sides of the merge. The problem can be solved by making the sync regions being a bit more powerful: we can keep a 0-length sync region if we find that a block unchanged by one side is ajacent to a block unchanged by the other side. Since these 0-length sync regions are emitted using the ~same algorithm as the normal non-empty sync regions, this change involves no arbitrary decisions and I expect it to work pretty well. 0-length sync regions do create an ambiguity in a special case where two pairs of adjacent regions "meet" at the same point. This corresponds to an insertion made at the same place by the two sides of the merge, and this still results in a conflict.