add: do not send if snapshots are equal

zfs-backup.py

@@ -50,7 +50,6 @@ def main():
 # TODO --skip-pre-backup-scrub
 # TODO --skip-post-backup-scrub
 # TODO print time estimation while scrub is in progress
-# skip send when sender and receiver snapshots are equal
 
 
 class Manager:
@@ -259,6 +258,8 @@ class ZFS:
     def send_incremental(old_snap: Snapshot, new_snap: Snapshot) -> Popen[str]:
         if old_snap.dataset != new_snap.dataset:
             raise ValueError(SAME_DATASET)
+        if old_snap == new_snap:
+            raise ValueError("Cannot send stream, snapshots are identical.")
         print(f"  {old_snap} -> {new_snap}")
         sender = Popen(
             [config.ZFS, "send", "-R", "-I", str(old_snap), str(new_snap)], stdout=PIPE