Split a large pull request into two

split-pull-requests.md

This is about how to split a GitHub pull request, following a real in-the-wild example.

So it happened that I once wanted to fix a bug (#832) on the shelljs repository.

I forked the repository, cloned it and created a new branch called ln-directory-dest. I fixed the bug, created a pull request, and implemented initial review feedback.

At this point, I had added the commits A, B, C, D, E and F (renamed here for simplicity). It made the repository look like this:

X -- [master]
 `- A -- B -- C -- D -- E -- F -- [ln-directory-dest]

However, it happened that the project maintainer encouraged me to take on some similar issues (namely, #829). I jumped on and fixed that one — and pushed the commit (G) to the ln-directory-dest branch.

X -- [master]
 `- A - B - C - D - E - F - G -- [ln-directory-dest]

I hoped this was okay, but the maintainer (understandibly) preferred having two separate pull requests for the fixes.

So, what was desired was this:

X -- [master]
 `- A - B - C - D - E - F -- [ln-directory-dest]
 `- G -- [ln-handle-dead-links]

So that left me wondering how to take G from the ln-directory-dest branch and put it onto ln-handle-dead-links.

After reading some articles (most about a similar git constellation, but not the same), I sucessfully trial-and-error'ed my way to success.

The steps are the following:

1. (Being on the ln-directory-dest branch,) create the new branch ln-handle-dead-links (but not checking it out):

   git branch ln-handle-dead-links

   Result:

   X -- [master]
    `- A - B - C - D - E - F - G -- [ln-directory-dest], [ln-handle-dead-links]
   

2. Set the HEAD of branch ln-directory-dest back by one commit (to F).

   git reset --hard HEAD~1

   Result:

   X -- [master]
    `- A - B - C - D - E - F -- [ln-directory-dest]
                           `- G -- [ln-handle-dead-links]
   

3. Rebase all commits that happened between branch ln-directory-dest and branch ln-handle-dead-links (which is only commit G) onto master.

   git rebase --onto master ln-directory-dest ln-handle-dead-links

   Result:

   X -- [master]
    `- A - B - C - D - E - F -- [ln-directory-dest]
    `- G -- [ln-handle-dead-links]
   

And we're done!