patch 8.2.1384: no ATTENTION prompt for :vimgrep first match file

Problem:    No ATTENTION prompt for :vimgrep first match file.
Solution:   When there is an existing swap file do not keep the dummy buffer.
            (closes #6649)
diff --git a/src/quickfix.c b/src/quickfix.c
index 1901e38..99b0169 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -6016,6 +6016,23 @@
 }
 
 /*
+ * Return TRUE if "buf" had an existing swap file, the current swap file does
+ * not end in ".swp".
+ */
+    static int
+existing_swapfile(buf_T *buf)
+{
+    if (buf->b_ml.ml_mfp != NULL)
+    {
+	char_u *fname = buf->b_ml.ml_mfp->mf_fname;
+	size_t len = STRLEN(fname);
+
+	return fname[len - 1] != 'p' || fname[len - 2] != 'w';
+    }
+    return FALSE;
+}
+
+/*
  * Search for a pattern in a list of files and populate the quickfix list with
  * the matches.
  */
@@ -6125,7 +6142,8 @@
 			buf = NULL;
 		    }
 		    else if (buf != *first_match_buf
-					|| (cmd_args->flags & VGR_NOJUMP))
+					|| (cmd_args->flags & VGR_NOJUMP)
+					|| existing_swapfile(buf))
 		    {
 			unload_dummy_buffer(buf, dirname_start);
 			// Keeping the buffer, remove the dummy flag.