Git merging one branch into another is a two way merge?

My git repo looks like this:

         _ branch_a
        /
master /
        \_ branch_b

Now I want to merge branch_b into branch_a, not either branch into master.

So, I did

git checkout branch_a
git merge branch_b

And git went and found a couple of conflicts.

When I do git mergetool using meld what I get on the console is

 {local}: modified file
 {remote}: modified file

and a window showing only LOCAL and REMOTE.

What I want to know is:

1. Why isn't the shared base of what was in master shown?

2. Between LOCAL and REMOTE, which one am I supposed to edit?