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/testdir/test_edit.vim b/src/testdir/test_edit.vim
index ab2fe7c..651f1f8 100644
--- a/src/testdir/test_edit.vim
+++ b/src/testdir/test_edit.vim
@@ -1375,6 +1375,14 @@
   set swapfile&
 endfunc
 
+func Test_edit_backtick()
+  next a\`b c
+  call assert_equal('a`b', expand('%'))
+  next
+  call assert_equal('c', expand('%'))
+  call assert_equal('a\`b c', expand('##'))
+endfunc
+
 func Test_edit_quit()
   edit foo.txt
   split