remote-curl: accept all encodings supported by curl

Configure curl to accept all encodings which curl supports instead of
only accepting gzip responses.

This fixes an issue when using an installation of curl which is built
without the "zlib" feature. Since aa90b9697 (Enable info/refs gzip
decompression in HTTP client, 2012-09-19) we end up requesting "gzip"
encoding anyway despite libcurl not being able to decode it.  Worse,
instead of getting a clear error message indicating so, we end up
falling back to "dumb" http, producing a confusing and difficult to
debug result.

Since curl doesn't do any checking to verify that it supports the a
requested encoding, instead set the curl option `CURLOPT_ENCODING` with
an empty string indicating that curl should send an "Accept-Encoding"
header containing only the encodings supported by curl.

Reported-by: Anton Golubev <anton.golubev@gmail.com>
Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.c

diff --git a/http.c b/http.c
index fed13b2..709150f 100644
--- a/http.c
+++ b/http.c
@@ -1788,7 +1788,7 @@ static int http_request(const char *url,
 
 	curl_easy_setopt(slot->curl, CURLOPT_URL, url);
 	curl_easy_setopt(slot->curl, CURLOPT_HTTPHEADER, headers);
-	curl_easy_setopt(slot->curl, CURLOPT_ENCODING, "gzip");
+	curl_easy_setopt(slot->curl, CURLOPT_ENCODING, "");
 
 	ret = run_one_slot(slot, &results);
 

remote-curl.c

diff --git a/remote-curl.c b/remote-curl.c
index ceb0534..565bba1 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -684,7 +684,7 @@ 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, "gzip");
+	curl_easy_setopt(slot->curl, CURLOPT_ENCODING, "");
 
 	if (large_request) {
 		/* The request body is large and the size cannot be predicted.

t/t5551-http-fetch-smart.sh

diff --git a/t/t5551-http-fetch-smart.sh b/t/t5551-http-fetch-smart.sh
index f5721b4..913089b 100755
--- a/t/t5551-http-fetch-smart.sh
+++ b/t/t5551-http-fetch-smart.sh
@@ -26,14 +26,14 @@
 cat >exp <<EOF
 > GET /smart/repo.git/info/refs?service=git-upload-pack HTTP/1.1
 > Accept: */*
-> Accept-Encoding: gzip
+> Accept-Encoding: ENCODINGS
 > 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: gzip
+> Accept-Encoding: ENCODINGS
 > Content-Type: application/x-git-upload-pack-request
 > Accept: application/x-git-upload-pack-result
 > Content-Length: xxx
@@ -79,8 +79,13 @@
 		/^< Date: /d
 		/^< Content-Length: /d
 		/^< Transfer-Encoding: /d
-	" >act &&
-	test_cmp exp act
+	" >actual &&
+	sed -e "s/^> Accept-Encoding: .*/> Accept-Encoding: ENCODINGS/" \
+			actual >actual.smudged &&
+	test_cmp exp actual.smudged &&
+
+	grep "Accept-Encoding:.*gzip" actual >actual.gzip &&
+	test_line_count = 2 actual.gzip
 '
 
 test_expect_success 'fetch changes via http' '