[RFC] git stash: add porcelain for sharing stashes through remotes

[Hanan Arshad <hananarshad619@gmail.com>]

Hi,

I'd like to propose adding a small porcelain workflow for sharing
stashes through a Git remote.

git stash export and git stash import already provide a transportable
representation of stashes. I tested the following workflow using
existing commands:

Alice:
  git stash export --print stash@{0}
  git push origin <export-tip>:refs/stashes/alice/wip

Bob:
  git fetch origin refs/stashes/alice/wip:refs/shared-stashes/origin/alice/wip
  git stash import refs/shared-stashes/origin/alice/wip

The imported stash is a normal local stash and retains the original
stash object ID. Removing the remote ref afterward does not affect
Bob's imported stash.

I'd like to add porcelain around this existing mechanism for four operations:

1. publish a selected stash to a remote
2. list available shared stashes
3. get a shared stash as a normal local stash
4. remove a shared stash from the remote

This would not introduce a new stash object format, server-side
service, or synchronization model. It would essentially compose the
existing export/import mechanism with normal push/fetch operations.
Before working on an implementation, I'd appreciate feedback on a few
design points:
1. What remote ref namespace would be appropriate?
2. Should shared stashes use an explicit user-provided name or an object-derived identifier?
3. Should listing only inspect remote refs, or fetch the export commits so stash messages can also be displayed?
4. What command naming would fit best with the existing git stash interface?

If the general direction seems reasonable, I can follow up with a more
concrete interface and implementation.

Thanks,
Hanan Arshad