Move sideband client side support into reusable form.

This moves the receiver side of the sideband support from
fetch-clone.c to sideband.c and its header file, so that
archiver protocol can use it.

Signed-off-by: Junio C Hamano <junkio@cox.net>

sideband.c

@@ -0,0 +1,48 @@
+#include "pkt-line.h"
+#include "sideband.h"
+
+/*
+ * Receive multiplexed output stream over git native protocol.
+ * in_stream is the input stream from the remote, which carries data
+ * in pkt_line format with band designator.  Demultiplex it into out
+ * and err and return error appropriately.  Band #1 carries the
+ * primary payload.  Things coming over band #2 is not necessarily
+ * error; they are usually informative message on the standard error
+ * stream, aka "verbose").  A message over band #3 is a signal that
+ * the remote died unexpectedly.  A flush() concludes the stream.
+ */
+int recv_sideband(const char *me, int in_stream, int out, int err, char *buf, int bufsz)
+{
+	while (1) {
+		int len = packet_read_line(in_stream, buf, bufsz);
+		if (len == 0)
+			break;
+		if (len < 1) {
+			len = sprintf(buf, "%s: protocol error: no band designator\n", me);
+			safe_write(err, buf, len);
+			return SIDEBAND_PROTOCOL_ERROR;
+		}
+		len--;
+		switch (buf[0] & 0xFF) {
+		case 3:
+			safe_write(err, "remote: ", 8);
+			safe_write(err, buf+1, len);
+			safe_write(err, "\n", 1);
+			return SIDEBAND_REMOTE_ERROR;
+		case 2:
+			safe_write(err, "remote: ", 8);
+			safe_write(err, buf+1, len);
+			continue;
+		case 1:
+			safe_write(out, buf+1, len);
+			continue;
+		default:
+			len = sprintf(buf + 1,
+				      "%s: protocol error: bad band #%d\n",
+				      me, buf[0] & 0xFF);
+			safe_write(err, buf+1, len);
+			return SIDEBAND_PROTOCOL_ERROR;
+		}
+	}
+	return 0;
+}