Don't clear scrollback with the \cl binding

ncurses since 6.0 sends the "E3" sequence along with "clear", even for
just `clear` or `tput clear`. This deletes the scrollback buffer which
is usually not what you want.

Fixes fish-shell#2855.

CHANGELOG.md

@@ -1,6 +1,7 @@
 # fish 2.6.0 (released ???)
 
 - The `read` command now has a default limit of 10 MiB. If a line is longer than that it will fail with $status set to 122 and the var will be empty. You can set a different limit by setting the FISH_READ_BYTE_LIMIT variable.
+- The \cl binding no longer deletes the scrollback buffer with ncurses >= 6.0 (#2855).
 
 ---
 

share/functions/__fish_shared_key_bindings.fish

@@ -77,7 +77,9 @@ function __fish_shared_key_bindings -d "Bindings shared between emacs and vi mod
 
     bind $argv \el __fish_list_current_token
     bind $argv \ew 'set tok (commandline -pt); if test $tok[1]; echo; whatis $tok[1]; commandline -f repaint; end'
-    bind $argv \cl 'clear; commandline -f repaint'
+    # ncurses > 6.0 sends a "delete scrollback" sequence along with clear.
+    # This string replace removes it.
+    bind $argv \cl 'clear | string replace \e\[3J ""; commandline -f repaint'
     bind $argv \cc __fish_cancel_commandline
     bind $argv \cu backward-kill-line
     bind $argv \cw backward-kill-path-component