patch 8.0.1777: cannot cleanup before loading another colorscheme

Problem:    Cannot cleanup before loading another colorscheme.
Solution:   Add the ColorSchemePre autocommand event.
diff --git a/src/fileio.c b/src/fileio.c
index 6b48c39..63cc61c 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -7734,6 +7734,7 @@
     {"CmdwinLeave",	EVENT_CMDWINLEAVE},
     {"CmdUndefined",	EVENT_CMDUNDEFINED},
     {"ColorScheme",	EVENT_COLORSCHEME},
+    {"ColorSchemePre",	EVENT_COLORSCHEMEPRE},
     {"CompleteDone",	EVENT_COMPLETEDONE},
     {"CursorHold",	EVENT_CURSORHOLD},
     {"CursorHoldI",	EVENT_CURSORHOLDI},
@@ -9479,7 +9480,8 @@
      */
     if (fname_io == NULL)
     {
-	if (event == EVENT_COLORSCHEME || event == EVENT_OPTIONSET)
+	if (event == EVENT_COLORSCHEME || event == EVENT_COLORSCHEMEPRE
+						   || event == EVENT_OPTIONSET)
 	    autocmd_fname = NULL;
 	else if (fname != NULL && !ends_excmd(*fname))
 	    autocmd_fname = fname;
@@ -9549,6 +9551,7 @@
 		|| event == EVENT_SPELLFILEMISSING
 		|| event == EVENT_QUICKFIXCMDPRE
 		|| event == EVENT_COLORSCHEME
+		|| event == EVENT_COLORSCHEMEPRE
 		|| event == EVENT_OPTIONSET
 		|| event == EVENT_QUICKFIXCMDPOST
 		|| event == EVENT_DIRCHANGED)