GeezerVison

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 <[email protected]>

Git removes each backslash-newline pair before parsing the resulting logical
value. Appending physical lines after processing only the first line left
trailing whitespace, comments, quotes, and recognized escapes literal.

That divergence returned values unlike git config and could preserve comment
text as configuration data.

- Accumulate continuation text before parsing it.
- Apply whitespace, comment, quote, and escape handling once to the complete value.
- Cover each affected syntax form with Git-compatible expectations.

Assisted-by: GPT 5.6
Co-authored-by: GPT 5.6 <[email protected]>