From: Derrick Stolee <derrickstolee@github.com>

When an object is not found in a repository's object store, we sometimes
call reprepare_packed_git() to see if the object was temporarily moved
into a new pack-file (and its old pack-file or loose object was
deleted). This process does a scan of each pack directory within each
odb, but does not reevaluate if the odb list needs updating.

Create a new reprepare_alt_odb() method that is a similar wrapper around
prepare_alt_odb(). Call it from reprepare_packed_git() under the object
read lock to avoid readers from interacting with a potentially
incomplete odb being added to the odb list.

prepare_alt_odb() already avoids adding duplicate odbs to the list
during its progress, so it is safe to call it again from
reprepare_alt_odb() without worrying about duplicate odbs.

This change is specifically for concurrent changes to the repository, so
it is difficult to create a test that guarantees this behavior is
correct. I manually verified by introducing a reprepare_packed_git() call
into get_revision() and stepped into that call in a debugger with a
parent 'git log' process. Multiple runs of reprepare_alt_odb() kept
the_repository->objects->odb as a single-item chain until I added a
.git/objects/info/alternates file in a different process. The next run
added the new odb to the chain and subsequent runs did not add to the
chain.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>

Submitted-As: https://lore.kernel.org/git/pull.1490.git.1678136369387.gitgitgadget@gmail.com