Re: 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?