fix: ignore continuation markers in config comments

<!-- agent -->
Git treats # and ; outside quotes as the start of a comment. A trailing
backslash in that ignored text therefore cannot continue the value.

The continuation loop counted trailing slashes without lexical context, so
it consumed the next option and a later writable flush silently dropped that
setting.

- Track quote and escape state while checking the accumulated value.
- Stop continuation scanning at an unquoted comment.
- Cover preservation of the following option across a writable flush.

Assisted-by: GPT 5.6
Co-authored-by: GPT 5.6 <codex@openai.com>

Author: Byron

git/config.py

@@ -467,6 +467,19 @@ def string_decode(v: str) -> str:
 
         # END string_decode
 
+        def is_line_continuation(value: str) -> bool:
+            quoted = escaped = False
+            for char in value:
+                if escaped:
+                    escaped = False
+                elif char == "\\":
+                    escaped = True
+                elif char == '"':
+                    quoted = not quoted
+                elif char in "#;" and not quoted:
+                    return False
+            return escaped
+
         while True:
             # We assume to read binary!
             line = fp.readline().decode(defenc)
@@ -520,8 +533,7 @@ def string_decode(v: str) -> str:
                         # included). An even number means the last backslash
                         # is escaped and the value ends there.
                         while True:
-                            trailing = len(optval) - len(optval.rstrip("\\"))
-                            if trailing % 2 == 0:
+                            if not is_line_continuation(optval):
                                 break
                             continuation = fp.readline()
                             if not continuation:

test/test_config.py

@@ -163,6 +163,19 @@ def test_backslash_line_continuation(self):
             key = "co" if section == "alias" else "k"
             self.assertEqual(config.get_value(section, key), expected)
 
+    @with_rw_directory
+    def test_comment_backslash_does_not_continue_value(self, rw_dir):
+        config_path = osp.join(rw_dir, "config")
+        with open(config_path, "wb") as config_file:
+            config_file.write(b"[a]\n\tk = one\\\n two ; ignored \\\n\tx = two\n")
+
+        with GitConfigParser(config_path, read_only=False) as config:
+            self.assertEqual(config.get_value("a", "x"), "two")
+            config.set_value("a", "added", "three")
+
+        with GitConfigParser(config_path) as config:
+            self.assertEqual(config.get_value("a", "x"), "two")
+
     def test_config_value_with_trailing_new_line(self):
         config_content = b'[section-header]\nkey:"value\n"'
         config_file = io.BytesIO(config_content)