patch 9.0.0396: :findrepl does not escape '&' and '~' properly

Problem:    :findrepl does not escape '&' and '~' properly.
Solution:   Escape depending on the value of 'magic'. (closes #11067)
diff --git a/src/gui.c b/src/gui.c
index 9c78dac..b9217b4 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -5360,8 +5360,10 @@
     if (type == FRD_REPLACEALL)
     {
 	ga_concat(&ga, (char_u *)"/");
-	// escape slash and backslash
-	p = vim_strsave_escaped(repl_text, (char_u *)"/\\");
+	// Escape slash and backslash.
+	// Also escape tilde and ampersand if 'magic' is set.
+	p = vim_strsave_escaped(repl_text,
+				p_magic ? (char_u *)"/\\~&" : (char_u *)"/\\");
 	if (p != NULL)
 	    ga_concat(&ga, p);
 	vim_free(p);