recreate: fix crash if recompress and deduplication beyond autocommit_threshold

ie. it means that if all the recompressed chunks were already in the repo
no data would be written, so there would be no active txn, so failure
ensues.

src/borg/archive.py

@@ -1450,7 +1450,7 @@ class ArchiveRecreater:
                 chunk_id, size, csize = self.cache.add_chunk(chunk_id, chunk, target.stats, overwrite=overwrite)
                 new_chunks.append((chunk_id, size, csize))
                 self.seen_chunks.add(chunk_id)
-                if self.recompress:
+                if self.recompress and self.cache.seen_chunk(chunk_id) == 1:
                     # This tracks how many bytes are uncommitted but compactable, since we are recompressing
                     # existing chunks.
                     target.recreate_uncomitted_bytes += csize