parseopt: restore const qualifier to parsed filename

This was unintentionally dropped in ccfcaf3 (parseopt: values of
pathname type can be prefixed with :(optional), 2025-09-28). Notably,
continue dropping the const qualifier when free'ing value; see
4049b9c (fix const issues with some functions, 2007-10-16) or
83838d5 (cast variable in call to free() in builtin/diff.c and
submodule.c, 2011-11-06) for more details on why.

Suggested-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: benknoble

parse-options.c

@@ -213,7 +213,7 @@ static enum parse_opt_result do_get_value(struct parse_opt_ctx_t *p,
 		if (unset)
 			value = NULL;
 		else if (opt->flags & PARSE_OPT_OPTARG && !p->opt)
-			value = (char *)opt->defval;
+			value = (const char *)opt->defval;
 		else {
 			int err = get_arg(p, opt, flags, &value);
 			if (err)