Make bracketed paste add only one undo entry

Bracketed paste adds one undo entry unless the pasted text contains a '
or \.  This is because the "paste" bind-mode has bindings for those keys,
so they effectively start a new undo entry.

Let's fix this by adding an explicit undo group (our first use of this
feature!).

share/functions/__fish_shared_key_bindings.fish

@@ -216,10 +216,12 @@ function __fish_start_bracketed_paste
     # we escape single-quotes (and backslashes).
     string match -q 'single*' (__fish_tokenizer_state -- (commandline -ct | string collect))
     and set -g __fish_paste_quoted 1
+    commandline -f begin-undo-group
 end
 
 function __fish_stop_bracketed_paste
     # Restore the last bind mode.
     set fish_bind_mode $__fish_last_bind_mode
     set -e __fish_paste_quoted
+    commandline -f end-undo-group
 end