Skip lines with spaces only in .gitignore

fix #878

dulwich/ignore.py

@@ -123,7 +123,7 @@ def read_ignore_patterns(f: BinaryIO) -> Iterable[bytes]:
         line = line.rstrip(b"\r\n")
 
         # Ignore blank lines, they're used for readability.
-        if not line:
+        if not line.strip():
             continue
 
         if line.startswith(b"#"):

dulwich/tests/test_ignore.py

@@ -105,7 +105,7 @@ class ReadIgnorePatterns(TestCase):
         f = BytesIO(
             b"""
 # a comment
-
+\x20\x20
 # and an empty line:
 
 \\#not a comment