How to merge from newer branch to older branches?

I'm not sure how to merge a topic branch onto multiple branches without conflicts or unwanted changes. My workflow probably isn't ideal, but I often run into cases like this and wondered if there's a way to solve it with git.

We have a "future" branch for new development. I wanted to add a new feature so I branched from "future" to create a branch named "feature". I completed the work, but now they've decided they need that on two older branches. It looks like this:

future o-o-o-o-o-o-o
        \   \       \
1.2          o-o-o   \
          \           \
1.1        o-o-o       \
                        \
feature                  o-o-o
                         a b c

I've read I should have started from the oldest common denominator branch that I might ever want, but at the time I created the branch, "future" was it because we thought 1.1 and 1.2 were locked down and not going to add new features. But things change and I need to roll my feature into 1.1 and 1.2.

What's the best way to merge changes on the "feature" branch into the 1.1 and 1.2 branches? If I try to checkout 1.1 then 'git merge feature', it (obviously) merges in a bunch of changes from "future" that I don't want (and has a bunch of conflicts). I also tried just cherry-picking commits a, b, and c from the "feature" branch. That mostly worked, but there were some surprising conflicts. Text that had been inserted into a couple files (which had been untouched in 1.1 and 1.2) caused a conflict when it seemed like it should have just added the lines without conflict. I realize it's difficult to know why it conflicted without seeing the changes, but I'm just asking in a more general way how I should be doing merges like this.

Is cherry-picking the best way? It seems pretty tedious because I have to do each commit in reverse order (although I could script it). I was hoping there was a way to say "apply all the changes from when "feature" branched up to its tip". I would also like to preserve the commit history rather than just making making one big commit of all the changes.