[Feature request] sync --abort-if / --max-deletes

--abort-if NUM

--abort-if 0
If there are no files present in source do not delete files in dest.

--abort-if 10
If there are less then 10 files in source do not delete files in dest.

rsync
max-delete=NUM
This tells rsync not to delete more than NUM files or directo‐
ries. If that limit is exceeded, all further deletions are
skipped through the end of the transfer. At the end, rsync out‐
puts a warning (including a count of the skipped deletions) and
exits with an error code of 25 (unless some more important error
condition also occurred).

          Beginning  with version 3.0.0, you may specify --max-delete=0 to
          be warned about any extraneous files in the destination  without
          removing any of them.  Older clients interpreted this as "unlim‐
          ited", so if you don’t know what version the client is, you  can
          use  the  less  obvious --max-delete=-1 as a backward-compatible
          way to specify that no deletions be allowed (though  really  old
          versions didn’t warn when the limit was exceeded).

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