environment: store comment_line_char as a string

environment: store comment_line_char as a string

We'd like to eventually support multi-byte comment prefixes, but the
comment_line_char variable is referenced in many spots, making the
transition difficult.

Let's start by storing the character in a NUL-terminated string. That
will let us switch code over incrementally to the string format, and we
can easily support the existing code with a macro wrapper (since we'll
continue to allow only a single-byte prefix, this will behave
identically).

Once all references to the "char" variable have been converted, we can
drop it and enable longer strings.

We'll still have to touch all of the spots that create or set the
variable in this patch, but there are only a few (reading the config,
and the "auto" character selector).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>