Theirs, minus what mine removed from base, plus what mine added to it, in
theirs' order with mine's additions after, so that two nodes merging the
same pair produce the same array.
Parameters
base: any
the attribute's values as this process last knew them
mine: any
the live values this process's change produced
theirs: any
the values currently in the store
Returns string[]
the merged values as strings, or undefined when none are left
(which the caller reads as the attribute being gone)
Merges one list-valued attribute three ways.
Theirs, minus what mine removed from base, plus what mine added to it, in theirs' order with mine's additions after, so that two nodes merging the same pair produce the same array.