merging 2 diverged branches creates a commit

git merge mybranch

Diagram of two boxes in a row, one with a heart, and one with a star. From the star, it branches out into a branch with a hash symbol, labelled main. The other branch coming off of the star has a box with a spiral followed by a box with a spiky symbol. The two branches converge in a box with a diamond symbol, labelled “merge commit!”.

merge commits have a few surprising gotchas!

gotcha: merging isn’t symmetric

normal:

git checkout main
git merge mybranch

weird:

git checkout mybranch
git merge main

these two result in the same code, but the merge commit’s parents have a different order

This comes up when you use HEAD^: it refers to the first parent, and usually you want that to be the commit from the main branch

gotcha: you can keep coding during a merge

If you forget you’re doing a merge, it’s easy to accidentally keep writing code and add a bunch of unrelated changes into the merge commit.

I use my prompt to remind me.

gotcha: git show doesn’t tell you what the merge commit did

It’ll often just show the merge commit as “empty” even if the merge did something important (like discard changes from one side).

Illustration of a tiny sad stick person with curly hair

person: why

tip: see what a merge did with git show --remerge-diff

git show --remerge-diff COMMIT_ID

will re-merge the parents and show you the difference between the original merge and what’s actually in the merge commit