runtime: Set b:undo_indent where missing (#12944)

Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/indent/fish.vim

@@ -3,6 +3,7 @@
 " Maintainer:   Nicholas Boyle (github.com/nickeb96)
 " Repository:   https://github.com/nickeb96/fish.vim
 " Last Change:  February 4, 2023
+"               2023 Aug 28 by Vim Project (undo_indent)
 
 if exists("b:did_indent")
     finish
@@ -12,6 +13,8 @@ let b:did_indent = 1
 setlocal indentexpr=GetFishIndent(v:lnum)
 setlocal indentkeys+==end,=else,=case
 
+let b:undo_indent = "setlocal indentexpr< indentkeys<"
+
 function s:PrevCmdStart(linenum)
     let l:linenum = a:linenum
     " look for the first line that isn't a line continuation