Merge branch 'sp/maint-http-enable-gzip' into maint

"git fetch" over http advertised that it supports "deflate", which
is much less common, and did not advertise more common "gzip" on its
Accept-Encoding header.

* sp/maint-http-enable-gzip:
  Enable info/refs gzip decompression in HTTP client

http.c

@@ -818,6 +818,7 @@ static int http_request(const char *url, void *result, int target, int options)
 
 	curl_easy_setopt(slot->curl, CURLOPT_URL, url);
 	curl_easy_setopt(slot->curl, CURLOPT_HTTPHEADER, headers);
+	curl_easy_setopt(slot->curl, CURLOPT_ENCODING, "gzip");
 
 	if (start_active_slot(slot)) {
 		run_active_slot(slot);

remote-curl.c

@@ -379,7 +379,7 @@ static int probe_rpc(struct rpc_state *rpc)
 	curl_easy_setopt(slot->curl, CURLOPT_NOBODY, 0);
 	curl_easy_setopt(slot->curl, CURLOPT_POST, 1);
 	curl_easy_setopt(slot->curl, CURLOPT_URL, rpc->service_url);
-	curl_easy_setopt(slot->curl, CURLOPT_ENCODING, "");
+	curl_easy_setopt(slot->curl, CURLOPT_ENCODING, NULL);
 	curl_easy_setopt(slot->curl, CURLOPT_POSTFIELDS, "0000");
 	curl_easy_setopt(slot->curl, CURLOPT_POSTFIELDSIZE, 4);
 	curl_easy_setopt(slot->curl, CURLOPT_HTTPHEADER, headers);
@@ -435,7 +435,7 @@ static int post_rpc(struct rpc_state *rpc)
 	curl_easy_setopt(slot->curl, CURLOPT_NOBODY, 0);
 	curl_easy_setopt(slot->curl, CURLOPT_POST, 1);
 	curl_easy_setopt(slot->curl, CURLOPT_URL, rpc->service_url);
-	curl_easy_setopt(slot->curl, CURLOPT_ENCODING, "");
+	curl_easy_setopt(slot->curl, CURLOPT_ENCODING, "gzip");
 
 	headers = curl_slist_append(headers, rpc->hdr_content_type);
 	headers = curl_slist_append(headers, rpc->hdr_accept);

t/t5551-http-fetch.sh

@@ -32,13 +32,14 @@ setup_askpass_helper
 cat >exp <<EOF
 > GET /smart/repo.git/info/refs?service=git-upload-pack HTTP/1.1
 > Accept: */*
+> Accept-Encoding: gzip
 > Pragma: no-cache
 < HTTP/1.1 200 OK
 < Pragma: no-cache
 < Cache-Control: no-cache, max-age=0, must-revalidate
 < Content-Type: application/x-git-upload-pack-advertisement
 > POST /smart/repo.git/git-upload-pack HTTP/1.1
-> Accept-Encoding: deflate, gzip
+> Accept-Encoding: gzip
 > Content-Type: application/x-git-upload-pack-request
 > Accept: application/x-git-upload-pack-result
 > Content-Length: xxx