screen: draw new content immediately, to prevent color flashes

After updating the color palette, the corresponding new window content
should be drawn immediately, before some other part of the code calls
doupdate(), to prevent the old content being shown in the new colors.

This fixes https://savannah.gnu.org/bugs/?49912.
Reported-by: Mike Frysinger <vapier@gentoo.org>

src/files.c

@@ -563,7 +563,8 @@ void display_buffer(void)
 	precalc_multicolorinfo();
 #endif
 
-    refresh_needed = TRUE;
+    /* Update the content of the edit window straightaway. */
+    edit_refresh();
 }
 
 #ifndef DISABLE_MULTIBUFFER