Improve handling comment characters in config file

Comment characters that are within balanced quotes should not discard
the rest of the line:

[branch "my#branch"]

is a valid section for a branch named my#branch (# is a valid
character in a branch name [0]). Previously this normalized to:

[branch "my

which raised an exception for finding an invalid section header.

Note that the current comment parser solution leaves ambiguity in case
of multi-byte encoded data in configuration files due to the risk of a
byte clashing with either of ", # or ; as they are represented in
ASCII, likely leading to interpreting a configuration file as malformed
depending on e.g. branch names. In real-life systems, the only
multi-byte encoding that is likely to be encountered for Git
configuration files is UTF-8, which should be safe from such accidental
clashes due to how it uses the high bit as continuation byte marker.

The same non-UTF-8 multi-byte issue seems to hold true for Git itself as
its current configuration parser is written [1], and it makes sense to
follow the reference implementation in this regard.

[0]: man git-check-ref-format
[1]: https://github.com/git/git/blob/c2ece9dc/config.c#L504