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)
diff --git a/src/fileio.c b/src/fileio.c
index 5e79870..1bf4d87 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -6742,6 +6742,8 @@
 #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 @@
 	    if (!n)
 	    {
 		if (*reason == 'd')
-		    mesg = _("E211: File \"%s\" no longer available");
+		{
+		    // Only give the message once.
+		    if (prev_b_mtime != -1)
+			mesg = _("E211: File \"%s\" no longer available");
+		}
 		else
 		{
 		    helpmesg = TRUE;