prepare-commit-msg hook during rebase

Hi there

For a while now, I thought I was using prepare-commit-msg hook wrong but finally took the time to analyse this further. Per the doc, the hook receives the source of the commit message as $2 (message/template/merge/squash/commit or empty).

I notice the following with `rebase -i`:
- When rb applies a patch to be edited/reworded, the hook is called
with `commit` as the message source.
- When rb applies a patch merely to be picked, the hook is called with
`message` as the message source.

The latter also happens when non-interactive rebase applies commits.

I find this confusing for two reasons:
- Whether edit/reword or pick, there is always a commit being applied,
and it's the source of the message. So why not `commit` in both cases?
- The doc says that `message` is for inputs from `-m` or `-F`. So,
certainly this should not apply when the message comes from a picked
commit.

Also, I'm not sure whether the claim about `-m` is true, but that's another issue; we even might want to distinguish between `-m` and `-F` here.

Does the source `message` during rb pick occur due to an implementation detail, maybe since the rewrite to sequencer?

Cheers Michael