chg: server exited with code 0 without being connectable is an error

Before this patch, if the server started by chg has exited with code 0 without
creating a connectable unix domain socket at the specified address, chg will
exit with code 0, which is not the correct behavior. It can happen, for
example, CHGHG is set to /bin/true.

This patch addresses the issue by checking the exit code of the server and
printing a new error message if the server exited normally but cannot be
reached.

contrib/chg/chg.c

@@ -269,6 +269,9 b' static hgclient_t *retryconnectcmdserver'
 
 cleanup:
 	if (WIFEXITED(pst)) {
+		if (WEXITSTATUS(pst) == 0)
+			abortmsg("could not connect to cmdserver "
+				 "(exited with status 0)");
 		debugmsg("cmdserver exited with status %d", WEXITSTATUS(pst));
 		exit(WEXITSTATUS(pst));
 	} else if (WIFSIGNALED(pst)) {