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)

src/ex_docmd.c

@@ -10954,7 +10954,7 @@ arg_all(void)
 #ifndef BACKSLASH_IN_FILENAME
 			    || *p == '\\'
 #endif
-			    )
+			    || *p == '`')
 		    {
 			/* insert a backslash */
 			if (retval != NULL)

src/testdir/test_edit.vim

@@ -1375,6 +1375,14 @@ func Test_edit_complete_very_long_name()
   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

src/version.c

@@ -798,6 +798,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    219,
 /**/
     218,
 /**/