http: drop const to fix strstr() warning

In redact_sensitive_header(), a C23 implementation of libc will complain
that strstr() assigns the result from "const char *cookie" to "char
*semicolon".

Ultimately the memory is writable. We're fed a strbuf, generate a const
pointer "sensitive_header" within it using skip_iprefix(), and then
assign the result to "cookie".  So we can solve this by dropping the
const from "cookie" and "sensitive_header".

However, this runs afoul of skip_iprefix(), which wants a "const char
**" for its out-parameter. We can solve that by teaching skip_iprefix()
the same "make sure out is at least as const as in" magic that we
recently taught to skip_prefix().

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

Author: peff

git-compat-util.h

@@ -902,8 +902,10 @@ static inline size_t xsize_t(off_t len)
  * is done via tolower(), so it is strictly ASCII (no multi-byte characters or
  * locale-specific conversions).
  */
-static inline bool skip_iprefix(const char *str, const char *prefix,
-			       const char **out)
+#define skip_iprefix(str, prefix, out) \
+	skip_iprefix_impl((str), (prefix), CONST_OUTPARAM((str), (out)))
+static inline bool skip_iprefix_impl(const char *str, const char *prefix,
+				     const char **out)
 {
 	do {
 		if (!*prefix) {

http.c

@@ -726,7 +726,7 @@ static int has_proxy_cert_password(void)
 static int redact_sensitive_header(struct strbuf *header, size_t offset)
 {
 	int ret = 0;
-	const char *sensitive_header;
+	char *sensitive_header;
 
 	if (trace_curl_redact &&
 	    (skip_iprefix(header->buf + offset, "Authorization:", &sensitive_header) ||
@@ -743,7 +743,7 @@ static int redact_sensitive_header(struct strbuf *header, size_t offset)
 	} else if (trace_curl_redact &&
 		   skip_iprefix(header->buf + offset, "Cookie:", &sensitive_header)) {
 		struct strbuf redacted_header = STRBUF_INIT;
-		const char *cookie;
+		char *cookie;
 
 		while (isspace(*sensitive_header))
 			sensitive_header++;