patch 8.1.0219: expanding ## fails to escape backtick

Problem:    Expanding ## fails to escape backtick.
Solution:   Escape a backtick in a file name. (closes #3257)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 4c653ff..54fb6ab 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -10954,7 +10954,7 @@
 #ifndef BACKSLASH_IN_FILENAME
 			    || *p == '\\'
 #endif
-			    )
+			    || *p == '`')
 		    {
 			/* insert a backslash */
 			if (retval != NULL)