It can be complicated to compute a diff in your head from the different sections in $MERGED and apply that. In my setup, meld helps by showing you these diffs visually, using:
[merge]
tool = mymeld
conflictstyle = diff3
[mergetool "mymeld"]
cmd = meld --diff $BASE $REMOTE --diff $REMOTE $LOCAL --diff $LOCAL $MERGED
It looks strange but offers a very convenient work-flow, using three tabs:
in tab 1 you see (from left to right) the change that you should make in tab 2 to solve the merge conflict.
in the right side of tab 2 you apply the "change that you should make" and copy the entire file contents to the clipboard (using ctrl-a and ctrl-c).
in tab 3 replace the right side with the clipboard contents. If everything is correct, you will now see - from left to right - the same change as shown in tab 1 (but with different contexts). Save the changes made in this tab.
Notes: