Locating merge that dropped a change

This morning, I was struggling (not for the first time) to produce a Git command that would identify a merge commit that dropped a change. I could see where it was added, but couldn't automate finding out why it wasn't any longer in HEAD.

All the permutations of "--full-history", "-m", "-S", "-G" on "git log" I could think of did not get me anywhere. As long as I had "--full-history", they could find the original commit that had added the change, but not the merge commit that had dropped it by taking the other parent.

So, how to automatically find a merge that ignored a known change?

And then for visualisation purposes, how do you persuade gitk's diff display to actually show that that merge commit removed the change from one of its parents? Again, "-m" didn't seem to work.

Help appreciated!

Kevin