fix: stop counting by default; the pass costs more than the transfer

The counting pass was added so the progress line could show a percentage and an
estimate, and on a real card it turned out to dominate the run. Measured
against the device: reading a track from the SMB mirror ran at 35 MB/s and
writing to the card at 21 MB/s, while the sync itself managed tens of kilobytes
per second. Neither end was slow. The cost was traversing fifty thousand files
across six thousand directories on FAT, and the counting pass does that a
second time, comparing both trees in full exactly as the transfer does.

Counting is now opt-in behind -P. Without it the progress line still shows the
running count, the transfer rate and the album in flight; the percentage and
the estimate are what needed the extra walk, and they were the least useful
part of the display.

That the fix for "it looks hung" was itself making it slow is the sort of thing
only measuring catches. The line still answers the question it was added for --
whether anything is happening -- without paying for the part that merely made
it prettier.

tests/test_sync_to_ipod.py

@@ -176,16 +176,16 @@ def test_the_help_says_how_to_reach_and_leave_disk_mode():
     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."""
+def test_counting_is_off_by_default(mirror, tmp_path):
+    """The counting pass walks and compares both trees exactly as the transfer
+    does. On a FAT card of fifty thousand files that costs more than moving the
+    data, so the percentage has to be asked for."""
     destination = tmp_path / "dest"
     destination.mkdir()
 
-    result = run("-f", "-S", "-U", "-Q", str(mirror), str(destination))
+    result = run("-f", "-S", "-U", 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()
 
@@ -314,7 +314,7 @@ def test_the_database_lands_at_the_device_root_not_the_music_folder(tmp_path):
     script = (
         f"mount --bind {card} {device} && "
         f"XDG_CACHE_HOME={tmp_path / 'cache'} MUSIC_MIRROR_DATABASE_TOOL={tool} "
-        f"bash {SCRIPT} -f -S -U -Q {tmp_path / 'mirror'} {device / 'Music'}"
+        f"bash {SCRIPT} -f -S -U {tmp_path / 'mirror'} {device / 'Music'}"
     )
     result = subprocess.run(
         ["unshare", "-Umr", "sh", "-c", script], capture_output=True, text=True
@@ -335,3 +335,14 @@ def test_the_database_lands_at_the_device_root_not_the_music_folder(tmp_path):
     # prefix, which is how the paths come out as /Music/... while the bytes are
     # read from somewhere else entirely.
     assert "Pendulum" in (scratch / "saw.txt").read_text()
+
+
+def test_counting_can_be_asked_for(mirror, tmp_path):
+    """When the destination is cheap to traverse, the percentage is worth it."""
+    destination = tmp_path / "dest"
+    destination.mkdir()
+
+    result = run("-f", "-S", "-U", "-P", str(mirror), str(destination))
+
+    assert result.returncode == 0, result.stderr
+    assert "files to copy" in result.stderr