Important: minimum fish version 3.2 -> 3.4 (c5e1707)

This is to make use of fish's set --function feature (see below). Additionally, I would like to keep the minimum version recent to enable simplifying assumptions about the environments that fzf.fish needs works on and reduce complexity.

Feature updates

[Search Git Status] use box drawing chars for diff type header (#291)

Make the header box for Git staged/unstaged/merged/etc. look nicer.
Old

+--------+
| Staged |
+--------+

New

╭────────╮
│ Staged │
╰────────╯

Function scope all variables to reduce side effects (cab67cc)

Without passing -f (--function), set commands in functions will clobber same-name shell variables that were already set. For example, if you do 'set commands_selected 1' and then execute _fzf_search_history, commands_selected will change value.

I chose -f over -l or a combination of the two to emulate Python's variable scopes, in which all variables declared within a function are function scoped. This makes reasoning about variables easier and more intuitive.