httpclient: use a 16kb read buffer for macOS

Use a 16kb read buffer for compatibility with macOS SecureTransport.

SecureTransport `SSLRead` has the following behavior:

1. It will return _at most_ one TLS packet's worth of data, and
2. It will try to give you as much data as you asked for

This means that if you call `SSLRead` with a buffer size that is smaller
than what _it_ reads (in other words, the maximum size of a TLS packet),
then it will buffer that data for subsequent calls.  However, it will
also attempt to give you as much data as you requested in your SSLRead
call.  This means that it will guarantee a network read in the event
that it has buffered data.

Consider our 8kb buffer and a server sending us 12kb of data on an HTTP
Keep-Alive session.  Our first `SSLRead` will read the TLS packet off
the network.  It will return us the 8kb that we requested and buffer the
remaining 4kb.  Our second `SSLRead` call will see the 4kb that's
buffered and decide that it could give us an additional 4kb.  So it will
do a network read.

But there's nothing left to read; that was the end of the data.  The
HTTP server is waiting for us to provide a new request.  The server will
eventually time out, our `read` system call will return, `SSLRead` can
return back to us and we can make progress.

While technically correct, this is wildly ineffecient.  (Thanks, Tim
Apple!)

Moving us to use an internal buffer that is the maximum size of a TLS
packet (16kb) ensures that `SSLRead` will never buffer and it will
always return everything that it read (albeit decrypted).

src/transports/httpclient.c

@@ -29,7 +29,18 @@ static git_http_auth_scheme auth_schemes[] = {
 	{ GIT_HTTP_AUTH_BASIC, "Basic", GIT_CREDENTIAL_USERPASS_PLAINTEXT, git_http_auth_basic },
 };
 
-#define GIT_READ_BUFFER_SIZE 8192
+/*
+ * Use a 16kb read buffer to match the maximum size of a TLS packet.  This
+ * is critical for compatibility with SecureTransport, which will always do
+ * a network read on every call, even if it has data buffered to return to
+ * you.  That buffered data may be the _end_ of a keep-alive response, so
+ * if SecureTransport performs another network read, it will wait until the
+ * server ultimately times out before it returns that buffered data to you.
+ * Since SecureTransport only reads a single TLS packet at a time, by
+ * calling it with a read buffer that is the maximum size of a TLS packet,
+ * we ensure that it will never buffer.
+ */
+#define GIT_READ_BUFFER_SIZE (16 * 1024)
 
 typedef struct {
 	git_net_url url;