objects: use object IDs and references instead of naive objs in status/transfer (#6360)

* objects.status: use object IDs instead of naive objects

* objects.transfer: use object IDs instead of naive objects

* objects: add ReferenceHashFile

* odb: add ReferenceObjectDB

* repo: collect used object IDs instead of naive objs

* support basic/naive fs serialization

* objects.stage: stage file objects in addition to trees

* objects: remove save(), update transfer()

* output/dep: update for osave removal

* path_info: support both url and path attributes

* status/gc: use object IDs instead of naive objs

* diff: update staging usage

* commit: update staging usage

* tests: clean staging when needed

* tests: update for save/transfer/stage changes

* dvcfs: don't allow config serialization (repofs should be used instead)

* utils: return mtime as str, size as int

* tests: update unit tests for save/stage/transfer changes

* objects: use Repo.open when needed in ReferenceHashFile

* fix ls test staging

* objects.stage: don't skip hash check when loading from state

- individual hashes will still be loaded from state if they are still
  valid

* ReferenceHashFile: serialize repo fs paths relative to repo root

* preserve state when staging

* objects.tree: skip hash computation in digest() when we have already
loaded hash from state

* always check file existence in referencehashfile

* update repofs test since stage no longer allows nonexistent files

* odb: only protect on check() if hash was verified

* staging: use unique memfs namespace for staging per dest ODB

* tests: clean full staging namespace

* refodb: preserve repofs cache dir

* repodependency: stage imports to local ODB and strict check circular
imports

* fetch: fetch staging last

* staging: make dest odb mandatory and preserve state

* refodb: cache nonlocal fs's

* fix rebase errors

* repofs: support full instantiation from fs config

* repofs: preserve subrepo factory for erepos

* tests: clarify repofs dirty file behavior

tests/utils/__init__.py

@@ -42,3 +42,15 @@ def dump_sv(stream, metrics, delimiter=",", header=True):
         writer = csv.writer(stream)
         for d in metrics:
             writer.writerow(list(d.values()))
+
+
+def clean_staging():
+    from dvc.fs.memory import MemoryFileSystem
+    from dvc.objects.stage import _STAGING_MEMFS_PATH
+
+    try:
+        MemoryFileSystem().fs.rm(
+            f"memory://{_STAGING_MEMFS_PATH}", recursive=True
+        )
+    except FileNotFoundError:
+        pass