http: honor no_http env variable to bypass proxy

Curl and its families honor several proxy related environment variables:

* http_proxy and https_proxy define proxy for http/https connections.
* no_proxy (a comma separated hosts) defines hosts bypass the proxy.

This command will bypass the bad-proxy and connect to the host directly:

    no_proxy=* https_proxy=http://bad-proxy/ \
    curl -sk https://google.com/

Before commit 372370f (http: use credential API to handle proxy auth...),
Environment variable "no_proxy" will take effect if the config variable
"http.proxy" is not set.  So the following comamnd won't fail if not
behind a firewall.

    no_proxy=* https_proxy=http://bad-proxy/ \
    git ls-remote https://github.com/git/git

But commit 372370f not only read git config variable "http.proxy", but
also read "http_proxy" and "https_proxy" environment variables, and set
the curl option using:

    curl_easy_setopt(result, CURLOPT_PROXY, proxy_auth.host);

This caused "no_proxy" environment variable not working any more.

Set extra curl option "CURLOPT_NOPROXY" will fix this issue.

Signed-off-by: Jiang Xin <xin.jiang@huawei.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.c

@@ -62,6 +62,7 @@ static long curl_low_speed_limit = -1;
 static long curl_low_speed_time = -1;
 static int curl_ftp_no_epsv;
 static const char *curl_http_proxy;
+static const char *curl_no_proxy;
 static const char *http_proxy_authmethod;
 static struct {
 	const char *name;
@@ -594,6 +595,11 @@ static CURL *get_curl_handle(void)
 		}
 
 		curl_easy_setopt(result, CURLOPT_PROXY, proxy_auth.host);
+#if LIBCURL_VERSION_NUM >= 0x071304
+		var_override(&curl_no_proxy, getenv("NO_PROXY"));
+		var_override(&curl_no_proxy, getenv("no_proxy"));
+		curl_easy_setopt(result, CURLOPT_NOPROXY, curl_no_proxy);
+#endif
 	}
 	init_curl_proxy_auth(result);