Don't check if output changed on first screen.

When invoked with the --chgexit/-g options wait until we have at
least one screen of output before checking if the output changed.
Otherwise, we're comparing the initial screen of output to all
spaces, which usually isn't what we want.

watch.c

@@ -657,7 +657,7 @@ int main(int argc, char *argv[])
 						tabpending = 0;
 				}
 				move(y, x);
-				if (!exit_early && option_chgexit) {
+				if (!first_screen && !exit_early && option_chgexit) {
 #ifdef WITH_WATCH8BIT
 					cchar_t oldc;
 					in_wch(&oldc);