[fish] Use FZF_DEFAULT_COMMAND for history

This makes the loading of the list non-blocking, since the command is
no longer piped to eval.

It also allows users to easily set key bindings via FZF_CTRL_R_OPTS,
that reload the history list. For example:

```
set -x FZF_CTRL_R_OPTS '--bind=\'ctrl-x:execute(history delete -- {2..})+reload(eval $FZF_DEFAULT_COMMAND)\''
```

To properly handle multi-line entries thought, the execute command of
the exapmle above, must become a little more complex:

```
eval history delete -- (string escape -- {2..} | string replace -a "\n\t" "\n")
```

Author: bitraid

shell/key-bindings.fish

@@ -69,16 +69,23 @@ function fzf_key_bindings
       if test "$FISH_MAJOR" -gt 2 -o \( "$FISH_MAJOR" -eq 2 -a "$FISH_MINOR" -ge 4 \)
         set -lx FZF_DEFAULT_OPTS (__fzf_defaults "" "-n2..,.. --scheme=history --bind=ctrl-r:toggle-sort --wrap-sign '"\t"↳ ' --highlight-line $FZF_CTRL_R_OPTS +m")
         set -lx FZF_DEFAULT_OPTS_FILE ''
+
+        # For compatibility with fish versions prior to 3.4.0, `set` should not
+        # use the `-f | --function` switch, so the FZF_DEFAULT_COMMAND variable
+        # is set as local, outside the `if` block.
+        set -lx FZF_DEFAULT_COMMAND
         if type -q perl
-          builtin history -z --reverse | command perl -0 -pe 's/^/$.\t/g; s/\n/\n\t/gm' | eval (__fzfcmd) --tac --read0 --print0 -q '(commandline)' | string replace -r '^\d*\t' '' | read -lz result
-          and commandline -- $result
+          set -a FZF_DEFAULT_OPTS '--tac'
+          set FZF_DEFAULT_COMMAND 'builtin history -z --reverse | command perl -0 -pe \'s/^/$.\t/g; s/\n/\n\t/gm\''
         else
-          set -l h (builtin history -z | string split0)
-          for i in (seq (count $h) -1 1)
-            string join0 -- $i\t(string replace -a -- \n \n\t $h[$i] | string collect)
-          end | eval (__fzfcmd) --read0 --print0 -q '(commandline)' | string replace -r '^\d*\t' '' | read -lz result
-          and commandline -- $result
+          set  FZF_DEFAULT_COMMAND \
+            'set -l h (builtin history -z --reverse | string split0);' \
+            'for i in (seq (count $h) -1 1);' \
+            'string join0 -- $i\t(string replace -a -- \n \n\t $h[$i] | string collect);' \
+            'end'
         end
+        eval (__fzfcmd) --read0 --print0 -q '(commandline)' | string replace -r '^\d*\t' '' | read -lz result
+        and commandline -- $result
       else
         builtin history | eval (__fzfcmd) -q '(commandline)' | read -l result
         and commandline -- $result