[RFC] worktree: add lifecycle hooks

[Maciej Ciemborowicz <maciej.ciemborowicz@gmail.com>]

Hello,

I'd like to discuss adding native hooks for worktree lifecycle events.

Git currently invokes `post-checkout` after `git worktree add` when a
checkout is performed. This makes it possible to detect some worktree
creation cases indirectly. There does not appear to be a native hook for
other worktree lifecycle operations such as:

    git worktree add
    git worktree remove
    git worktree move
    git worktree lock
    git worktree unlock
    git worktree prune

As a result, tools which want to react to these operations have to wrap
`git worktree` or compare the worktree state before and after a command.
They cannot observe an invocation which bypasses their wrapper.

One concrete use case is git-hooks-ext, which provides higher-level events
for Git operations. It currently has to implement worktree events by
wrapping `git worktree` and comparing the output of `git worktree list
--porcelain` before and after the command.

Would it make sense for Git to expose worktree lifecycle events directly?

One possibility would be separate post-operation hooks such as:

    post-worktree-add
    post-worktree-remove
    post-worktree-move
    post-worktree-lock
    post-worktree-unlock

Another possibility would be a single hook, similar in spirit to
`reference-transaction`, which reports worktree lifecycle changes. An
illustrative payload could look like:

    add    <path>
    remove <path>
    move   <old-path> <new-path>
    lock   <path>
    unlock <path>
    prune  <path>

A single hook may be useful for operations such as `git worktree prune`,
where one command can remove multiple administrative entries. It could
also leave room for `git worktree repair`, although I am unsure whether
repair belongs to the same interface.

For the use case I have in mind, these hooks would only need to notify
observers after a successful operation. They would not need to veto the
operation.

Before attempting an implementation, I'd like to know whether this would
fit Git's hook model, and whether a single lifecycle hook or individual
operation hooks would be preferable. I would also appreciate guidance on
whether a stable worktree identifier should be exposed in addition to the
path.

Thanks,
Maciej Ciemborowicz