[utils] Streap leading dots

Fixes #2865, closes #5087

test/test_utils.py

@@ -85,8 +85,11 @@ class TestUtil(unittest.TestCase):
         self.assertEqual(
             sanitize_filename('New World record at 0:12:34'),
             'New World record at 0_12_34')
+
         self.assertEqual(sanitize_filename('--gasdgf'), '_-gasdgf')
         self.assertEqual(sanitize_filename('--gasdgf', is_id=True), '--gasdgf')
+        self.assertEqual(sanitize_filename('.gasdgf'), 'gasdgf')
+        self.assertEqual(sanitize_filename('.gasdgf', is_id=True), '.gasdgf')
 
         forbidden = '"\0\\/'
         for fc in forbidden:

youtube_dl/utils.py

@@ -305,6 +305,7 @@ def sanitize_filename(s, restricted=False, is_id=False):
             result = result[2:]
         if result.startswith('-'):
             result = '_' + result[len('-'):]
+        result = result.lstrip('.')
         if not result:
             result = '_'
     return result