builtin/stash: provide a way to export stashes to a ref

A common user problem is how to sync in-progress work to another
machine.  Users currently must use some sort of transfer of the working
tree, which poses security risks and also necessarily causes the index
to become dirty.  The experience is suboptimal and frustrating for
users.

A reasonable idea is to use the stash for this purpose, but the stash is
stored in the reflog, not in a ref, and as such it cannot be pushed or
pulled.  This also means that it cannot be saved into a bundle or
preserved elsewhere, which is a problem when using throwaway development
environments.

In addition, users often want to replicate stashes across machines, such
as when they must use multiple machines or when they use throwaway dev
environments, such as those based on the Devcontainer spec, where they
might otherwise lose various in-progress work.

Let's solve this problem by allowing the user to export the stash to a
ref (or, to just write it into the repository and print the hash, à la
git commit-tree).  Introduce git stash export, which writes a chain of
commits where the first parent is always a chain to the previous stash,
or to a single, empty commit (for the final item) and the second is the
stash commit normally written to the reflog.

Iterate over each stash from top to bottom, looking up the data for each
one, and then create the chain from the single empty commit back up in
reverse order.  Generate a predictable empty commit so our behavior is
reproducible.  Create a useful commit message, preserving the author and
committer information, to help users identify stash commits when viewing
them as normal commits.

If the user has specified specific stashes they'd like to export
instead, use those instead of iterating over all of the stashes.

As part of this, specifically request quiet behavior when looking up the
OID for a revision because we will eventually hit a revision that
doesn't exist and we don't want to die when that occurs.

When exporting stashes, be sure to verify that they look like valid
stashes and don't contain invalid data.  This will help avoid failures
on import or problems due to attempting to export invalid refs that are
not stashes.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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