Estimate progress while rsync scans from the last backup's file count

With incremental recursion rsync only knows the total near the end, so the bar showed
no ETA for most of the backup. Save the file count as a third line of the state file
and use it as an estimate until rsync reports the real total.

tests/test_local.py

@@ -207,6 +207,19 @@ class TestRsync:
         assert 'rsync: total size' in caplog.text
         assert '%' not in caplog.text
 
+    def test_file_count(self, src: Path, out: Path) -> None:
+        backup = make_backup(out, [str(src)], progress=True)
+
+        assert backup.run() == 0
+        # The source folder, its two files and the implied parent folders
+        assert backup.file_count is not None and backup.file_count >= 3
+
+    def test_no_file_count_without_progress(self, src: Path, out: Path) -> None:
+        backup = make_backup(out, [str(src)])
+
+        assert backup.run() == 0
+        assert backup.file_count is None
+
     def test_cli(self, src: Path, out: Path, tmp_path: Path, run_cli: Callable[..., int]) -> None:
         config_file = str(tmp_path / 'missing.conf')
 
@@ -280,6 +293,30 @@ class TestProgressBar:
         assert bar.update('          3.00M   2%    1.47MB/s    0:00:03')
         assert capsys.readouterr().err.endswith(']   50%  2/4 files  3.00M copied  ETA 0:00:30\033[K')
 
+    def test_estimate_while_scanning(self, monkeypatch: pytest.MonkeyPatch,
+                                     capsys: pytest.CaptureFixture[str]) -> None:
+        monkeypatch.setattr(sb, 'get_terminal_size', lambda: os.terminal_size((100, 24)))
+        bar = sb._ProgressBar(estimate=4000)
+
+        assert bar.update('          2.00M  25%    1.47MB/s    0:00:02 (xfr#1, ir-chk=1000/2000)')
+        assert bar.total is None
+        assert capsys.readouterr().err == \
+            '\r[#######-----------------------]  ~25%  1,000/~4,000 files  2.00M copied  ETA ~0:01:30\033[K'
+
+    def test_estimate_exceeded(self, capsys: pytest.CaptureFixture[str]) -> None:
+        bar = sb._ProgressBar(estimate=1000)
+
+        # More files than in the last backup: back to plain scanning
+        assert bar.update('          2.00M  25%    1.47MB/s    0:00:02 (xfr#1, ir-chk=500/2000)')
+        assert capsys.readouterr().err == '\r1,500 files checked  2.00M copied  (scanning)\033[K'
+
+    def test_real_total_replaces_estimate(self, capsys: pytest.CaptureFixture[str]) -> None:
+        bar = sb._ProgressBar(estimate=4000)
+
+        assert bar.update('      4,000,000   1%    2.97MB/s    0:00:01 (xfr#2, to-chk=1/4)')
+        assert bar.total == 4
+        assert '  75%  3/4 files' in capsys.readouterr().err
+
     def test_not_progress(self, capsys: pytest.CaptureFixture[str]) -> None:
         bar = sb._ProgressBar()