fix memory leak parsing core.commentchar

When we see the core.commentchar config option, we extract the string with git_config_string, which does two things: 1. It complains via config_error_nonbool if there is no string value. 2. It makes a copy of the string. Since we immediately parse the string into its single-character value, we only care about (1). And in fact (2) is a detriment, as it means we leak the copy. Instead, let's just check the pointer value ourselves, and parse directly from the const string we already have. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>