patch 8.1.1780: warning for file no longer available is repeated

Problem:    Warning for file no longer available is repeated every time Vim is
            focused. (Brian Armstrong)
Solution:   Only give the message once. (closes #4748)

src/fileio.c

@@ -6742,6 +6742,8 @@ buf_check_timestamp(
 #endif
 		))
     {
+	long prev_b_mtime = buf->b_mtime;
+
 	retval = 1;
 
 	// set b_mtime to stop further warnings (e.g., when executing
@@ -6819,7 +6821,11 @@ buf_check_timestamp(
 	    if (!n)
 	    {
 		if (*reason == 'd')
+		{
+		    // Only give the message once.
+		    if (prev_b_mtime != -1)
 			mesg = _("E211: File \"%s\" no longer available");
+		}
 		else
 		{
 		    helpmesg = TRUE;

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1780,
 /**/
     1779,
 /**/