I'm struggling to follow why a merge base makes a difference? Or even exactly what one is?
A couple of questions which might help clear things up:
Do you consider `main` a branch?
If you create a new branch called `feature1`, do you consider the commits from before the branch from `main` to be "part of" that branch or not?
What if you delete `main`? Are the commits from before the branch part of the `feature1` branch then? What if there are multiple surviving feature branches that share varying amounts of common history?
main is a branch. By convention, it contains all of the commits back to an ancestor that has no parents.
Unless given more information, I would consider the commits between `git merge-base main feature1` (exclusive) and feature1 (inclusive) as part of the feature1 branch.
Now, if I `git checkout -b feature1-A feature1`, what commits are part of branch feature1-A? It depends. With respect to which merge base?
A couple of questions which might help clear things up:
Do you consider `main` a branch?
If you create a new branch called `feature1`, do you consider the commits from before the branch from `main` to be "part of" that branch or not?
What if you delete `main`? Are the commits from before the branch part of the `feature1` branch then? What if there are multiple surviving feature branches that share varying amounts of common history?