The original idea was to flatten left/right conflict trees and pair up adjacent
negative/positive terms. For example, diff(A, B-C+D) could be rendered as
diff(A, B) and diff(C, D). The problem of this formalization is that one of the
diff pairs is often empty (because e.g. A=B), so the context is fully omitted.
The resulting diff(C, D) doesn't provide any notion why the hunk is conflicted,
and how it is different from A.
Instead, this patch implements diffs in which each left/right pair is compared.
In the example above, the left terms are padded, and the diffs are rendered as
diff(A, B), diff(-A, -C), diff(A, D). This appears to be working reasonably well
so long as either side is resolved or both sides have the same numbers of terms.
Closes#4062
b63ab33b10