remote-curl: accept all encodings supported by curl

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>