ssh: read from stderr if stdout is empty

When we fail to read from stdout, it's typically because the URL was
wrong and the server process has sent some output over its stderr
output.

Read that output and set the error message to whatever we read from it.

CHANGELOG.md

@@ -45,6 +45,10 @@ support for HTTPS connections insead of OpenSSL.
 * The index now uses diffs for `add_all()` and `update_all()` which
   gives it a speed boost and closer semantics to git.
 
+* The ssh transport now reports the stderr output from the server as
+  the error message, which allows you to get the "repository not
+  found" messages.
+
 
 ### API additions
 

src/transports/ssh.c

@@ -125,10 +125,17 @@ static int ssh_stream_read(
 		return -1;
 
 	if ((rc = libssh2_channel_read(s->channel, buffer, buf_size)) < LIBSSH2_ERROR_NONE) {
-		ssh_error(s->session, "SSH could not read data");;
+		ssh_error(s->session, "SSH could not read data");
 		return -1;
 	}
 
+	/* Having something in stderr is typically a not-found error */
+	if (rc == 0 && (rc = libssh2_channel_read_stderr(s->channel, buffer, buf_size)) > 0) {
+		giterr_set(GITERR_SSH, "%*s", rc, buffer);
+		return -1;
+	}
+
+
 	*bytes_read = rc;
 
 	return 0;