Redo part of an already committed merge

Question

I have committed a big merge. Later, it appears that some files weren\'t merged correctly. I want to redo just these files, not the entire merge - the merge was big and I do

Answer 1

You can, in fact, simply perform the merge again. (Be sure you have git rerere turned off.)

Remember, git merge looks at three things: your current commit, the commit you ask it to merge, and the merge-base of those two commits. (Well, also your strategy arguments and so on, of course, but you can repeat those as well.) It's true that you already committed a merge result, but you can still get back on the commit you were on earlier:

          o---o---X       <-- otherbranch
         /         \
...--o--*           \ 
         \           \
          o--o--o--Y--M   <-- HEAD -> yourbranch

Your merge M is the result of merging commits Y and X (with merge base *). But you can check out commit Y as either a detached HEAD or a new branch. To make a new branch:

git checkout -b newbranch yourbranch^

which produces:

          o---o----X      <-- otherbranch
         /          \
...--o--*            M    <-- yourbranch
         \          /
          o--o--o--Y      <-- HEAD -> newbranch

(this is the exact same graph but I moved M up a bit to make room for newbranch to point to commit Y—think of the graph as being kind of rubbery/stretchy, or printed on Play-Doh®, or whatever). Now you can git merge otherbranch and start re-creating commit M.

Now you can grab merge results (rather than re-resolving) using git checkout yourbranch -- path for various paths (even the top path), and optionally git checkout -m parts to re-create conflicts.

When you are all done, git commit will make a new M2 merge whose first parent is Y and second parent is X, and newbranch will point to the new merge M2.