fix: join backslash line continuations when reading config values

git config joins an unquoted value that ends in a backslash with the
next line: the backslash and the newline are removed and the next line
is appended verbatim, so 'k = line1\' followed by ' line2' reads back
as 'line1 line2'. GitPython only implemented multi-line values for
quoted strings; for unquoted values the continuation line was silently
dropped, truncating whatever was stored in the config.

Read the continuation inside _read and join it, chaining across lines
that themselves end in a backslash. An even number of trailing
backslashes is an escaped one, so the value ends there; a single
backslash right at end-of-file is dropped, matching git. Verified
against git itself for every case covered by the new tests.

Author: NK

git/config.py

@@ -513,7 +513,26 @@ def string_decode(v: str) -> str:
 
                     if len(optval) < 2 or optval[0] != '"':
                         # Does not open quoting.
-                        pass
+                        # A value ending in an odd number of backslashes
+                        # continues on the next line, exactly as git does: the
+                        # final backslash and the newline are removed and the
+                        # next line is appended verbatim (leading whitespace
+                        # 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:
+                                break
+                            continuation = fp.readline()
+                            if not continuation:
+                                # Backslash at end of file: git drops it.
+                                optval = optval[:-1]
+                                break
+                            lineno = lineno + 1
+                            joined = continuation.decode(defenc)
+                            while joined.endswith("\n") or joined.endswith("\r"):
+                                joined = joined[:-1]
+                            optval = optval[:-1] + joined
                     elif optval[-1] != '"':
                         # Opens quoting and does not close: appears to start multi-line quoting.
                         is_multi_line = True

test/test_config.py

@@ -143,6 +143,26 @@ def test_multi_line_config(self):
             )
             self.assertEqual(len(config.sections()), 23)
 
+    def test_backslash_line_continuation(self):
+        """An unquoted value ending in a backslash continues on the next line,
+        exactly as git config parses it: the final backslash and the newline
+        are removed and the next line is appended verbatim."""
+        cases = [
+            (b"[a]\n\tk = line1\\\n line2\n", "line1 line2"),
+            (b"[a]\n\tk = one\\\n two\\\n three\n", "one two three"),
+            (b"[a]\n\tk = val\\\\\n next\n", "val\\\\"),
+            (b"[a]\n\tk = end\\\n", "end"),
+            (b"[alias]\n\tco = checkout \\\n\t\t-v\n", "checkout \t\t-v"),
+        ]
+        for content, expected in cases:
+            config_file = io.BytesIO(content)
+            config_file.name = "backslash_continuation.config"
+            config = GitConfigParser(config_file)
+            config.read()
+            section = "alias" if b"[alias]" in content else "a"
+            key = "co" if section == "alias" else "k"
+            self.assertEqual(config.get_value(section, key), expected)
+
     def test_config_value_with_trailing_new_line(self):
         config_content = b'[section-header]\nkey:"value\n"'
         config_file = io.BytesIO(config_content)