Create a --retries-failed-only option

As stated in the documentation, the --retries int option "Retries the entire sync", which requires a lot of useless work on both sides for just few files having failed on a very big tree.

Would it be possible to create a --retries-failed-only int option ?

I imagine it working like this :

• each time a file transfer fails, add its source and destination paths in a list kept in memory
• at the end of the sync, retry only the content of this list instead of retrying the entire sync
  Of course this list could be limited to, for example, 100 failing files. If the list reaches this limit, then a full sync is triggered instead.

In a sync I am currently running, I see 10 or 20 errors for more than 100.000 files being processed, which means that the next retry will check all these files again whereas it should, in my opinion, only retry the failed ones.

Start by reading the documentation for the existing --retries option and tracing its implementation through sync failure handling. Define how failed source and destination paths are retained, how the 100-file limit triggers a full sync, and verify that retries process only recorded failures when the limit is not reached.