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.

<!-- 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>