rebuild_archives_directory: accelerate by only reading metadata

We are only interested in archive metadata objects here, thus for most repo objects
it is enough to read the repoobj's metadata and determine the object's type.

Only if it is the right type of object, we need to read the full object (metadata
and data).

src/borg/archive.py

@@ -1835,6 +1835,16 @@ def valid_archive(obj):
         )
         for chunk_id, _ in self.chunks.iteritems():
             pi.show()
+            cdata = self.repository.get(chunk_id, read_data=False)  # only get metadata
+            try:
+                meta = self.repo_objs.parse_meta(chunk_id, cdata, ro_type=ROBJ_DONTCARE)
+            except IntegrityErrorBase as exc:
+                logger.error("Skipping corrupted chunk: %s", exc)
+                self.error_found = True
+                continue
+            if meta["type"] != ROBJ_ARCHIVE_META:
+                continue
+            # now we know it is an archive metadata chunk, load the full object from the repo:
             cdata = self.repository.get(chunk_id)
             try:
                 meta, data = self.repo_objs.parse(chunk_id, cdata, ro_type=ROBJ_DONTCARE)
@@ -1843,7 +1853,7 @@ def valid_archive(obj):
                 self.error_found = True
                 continue
             if meta["type"] != ROBJ_ARCHIVE_META:
-                continue
+                continue  # should never happen
             try:
                 archive = msgpack.unpackb(data)
             # Ignore exceptions that might be raised when feeding msgpack with invalid data