http_request: reset "type" strbuf before adding

Callers may pass us a strbuf which we use to record the
content-type of the response. However, we simply appended to
it rather than overwriting its contents, meaning that cruft
in the strbuf gave us a bogus type. E.g., the multiple
requests triggered by http_request could yield a type like
"text/plainapplication/x-git-receive-pack-advertisement".

Reported-by: Michael Schubert <mschub@elegosoft.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.c

@@ -841,6 +841,7 @@ static int http_request(const char *url, struct strbuf *type,
 
 	if (type) {
 		char *t;
+		strbuf_reset(type);
 		curl_easy_getinfo(slot->curl, CURLINFO_CONTENT_TYPE, &t);
 		if (t)
 			strbuf_addstr(type, t);