Teach upload-pack to log the received need lines to an fd

To facilitate testing and verification of the requests sent by
git-fetch to the remote side we permit logging the received packet
lines to the file descriptor specified in GIT_DEBUG_SEND_PACK has
been set.  Special start and end lines are included to indicate
the start and end of each connection.

  $ GIT_DEBUG_SEND_PACK=3 git fetch 3>UPLOAD_LOG
  $ cat UPLOAD_LOG
  #S
  want 8e10cf4e007ad7e003463c30c34b1050b039db78 multi_ack side-band-64k thin-pack ofs-delta
  want ddfa4a33562179aca1ace2bcc662244a17d0b503
  #E
  #S
  want 3253df4d1cf6fb138b52b1938473bcfec1483223 multi_ack side-band-64k thin-pack ofs-delta
  #E

>From the above trace the first connection opened by git-fetch was to
download two refs (with values 8e and dd) and the second connection
was opened to automatically follow an annotated tag (32).

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

upload-pack.c

@@ -35,6 +35,7 @@ static unsigned int timeout;
  * otherwise maximum packet size (up to 65520 bytes).
  */
 static int use_sideband;
+static int debug_fd;
 
 static void reset_timeout(void)
 {
@@ -444,13 +445,17 @@ static void receive_needs(void)
 	static char line[1000];
 	int len, depth = 0;
 
+	if (debug_fd)
+		write_in_full(debug_fd, "#S\n", 3);
 	for (;;) {
 		struct object *o;
 		unsigned char sha1_buf[20];
 		len = packet_read_line(0, line, sizeof(line));
 		reset_timeout();
 		if (!len)
 			break;
+		if (debug_fd)
+			write_in_full(debug_fd, line, len);
 
 		if (!prefixcmp(line, "shallow ")) {
 			unsigned char sha1[20];
@@ -506,6 +511,8 @@ static void receive_needs(void)
 			add_object_array(o, NULL, &want_obj);
 		}
 	}
+	if (debug_fd)
+		write_in_full(debug_fd, "#E\n", 3);
 	if (depth == 0 && shallows.nr == 0)
 		return;
 	if (depth > 0) {
@@ -631,6 +638,8 @@ int main(int argc, char **argv)
 		die("'%s': unable to chdir or not a git archive", dir);
 	if (is_repository_shallow())
 		die("attempt to fetch/clone from a shallow repository");
+	if (getenv("GIT_DEBUG_SEND_PACK"))
+		debug_fd = atoi(getenv("GIT_DEBUG_SEND_PACK"));
 	upload_pack();
 	return 0;
 }