diffconflicts [dc] lets you resolve diffs as a two way diff between what's in the conflict markers instead of including the resolved parts in the diff. It opens the diff in vim but could be adapted for other editors. Verbose explanation: https://github.com/whiteinge/diffconflicts/blob/master/READM...
The author converted it to a vim plugin with the same name, but I use a different vim plugin implementation [mergetool].