strbuf: avoid shadowing global comment_line_char name

strbuf: avoid shadowing global comment_line_char name

Several comment-related strbuf functions take a comment_line_char
parameter. There's also a global comment_line_char variable, which is
closely related (most callers pass it in as this parameter). Let's avoid
shadowing the global name. This makes it more obvious that we're not
using the global value, and it will be especially helpful as we refactor
the global in future patches (in particular, any macro trickery wouldn't
work because the preprocessor doesn't respect scope).

We'll use "comment_prefix". That should be descriptive enough, and as a
bonus is more neutral with respect to the "char" type (since we'll
eventually swap it out for a string).

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