perf: cut round trips over a network mount, and allow skipping the count

The transfer is metadata-bound rather than throughput-bound. Fifty thousand
files is fifty thousand round trips, and the counting pass added for the
percentage doubles that.

--whole-file is already implied when both ends are local paths, which an SMB or
FAT mount is, but stating it records that the delta algorithm is deliberately
unwanted here: it would read every destination file back over USB to checksum
it, in order to avoid resending an MP3 that has changed in its entirety anyway.

--omit-dir-times drops one setattr per directory. Across six thousand album
folders on a FAT card that is six thousand operations spent on timestamps
nothing reads.

-Q skips the counting pass. The percentage and the estimate are worth a second
walk of a local tree and frequently are not worth one of a network mount, so
that is now a choice rather than a fixed cost.

The README covers the part that is not an rsync flag at all: SMB defaults to a
one second attribute cache, so nearly every stat goes to the wire, twice. An
actimeo of sixty on the mount does more than any of the above, and closes most
of the gap that would otherwise argue for moving to NFS.

tests/test_sync_to_ipod.py

@@ -174,3 +174,33 @@ def test_the_help_says_how_to_reach_and_leave_disk_mode():
 
     assert "Menu+Select" in help_text
     assert "holding Play" in help_text
+
+
+def test_quick_mode_skips_the_counting_pass(mirror, tmp_path):
+    """Over SMB the walk is the expensive part, and doing it twice for a
+    percentage is not always the trade you want."""
+    destination = tmp_path / "dest"
+    destination.mkdir()
+
+    result = run("-f", "-S", "-U", "-Q", str(mirror), str(destination))
+
+    assert result.returncode == 0, result.stderr
+    assert "skipping the count" in result.stderr
+    assert "files to copy" not in result.stderr
+    assert (destination / "Album" / "track.mp3").is_file()
+
+
+def test_the_delta_algorithm_is_disabled(mirror, tmp_path):
+    """It would read every destination file back over USB to checksum it, to
+    avoid resending an MP3 that has changed in its entirety anyway."""
+    script = SCRIPT.read_text()
+
+    assert "--whole-file" in script
+
+
+def test_directory_timestamps_are_not_set(mirror, tmp_path):
+    """One setattr round trip per directory, across six thousand albums, to set
+    timestamps nothing reads."""
+    script = SCRIPT.read_text()
+
+    assert "--omit-dir-times" in script