Make hybrid bindings easier to achieve

The vi-bindings function would unconditionally erase all bindings,
making it impossible to call it last. This would disable the
mode-indicator (and in future also the cursor).

Make it so any argument to fish_vi_key_bindings stops it from erasing
bindings.

It would also be possible to demand an argument to erase (or to erase as
a separate step). but the usual case seems to be _switching_ to a set of bindings.

doc_src/index.hdr.in

@@ -1025,6 +1025,19 @@ You can change these key bindings using the <a href="commands.html#bind">bind</a
 
 Vi mode allows for the use of Vi-like commands at the prompt. Initially, <a href="#vi-mode-insert">insert mode</a> is active. @key{Escape} enters <a href="#vi-mode-command">command mode</a>. The commands available in command, insert and visual mode are described below. Vi mode shares <a href="#shared-binds">some bindings</a> with <a href="#emacs-mode">Emacs mode</a>.
 
+It is also possible to add all emacs-mode bindings to vi-mode by using something like
+
+\fish
+function fish_user_key_bindings
+    # Execute this once per mode that emacs bindings should be used in
+    fish_default_key_bindings -M insert
+    # Without an argument, fish_vi_key_bindings will default to
+    # resetting all bindings.
+    # The argument specifies the initial mode (insert, "default" or visual).
+    fish_vi_key_bindings insert
+end
+\endfish
+
 When in vi-mode, the <a href="fish_mode_prompt.html">`fish_mode_prompt`</a> function will display a mode indicator to the left of the prompt. The `fish_vi_cursor` function is available to change the cursor's shape depending on the mode in supported terminals.
 
 \subsubsection vi-mode-command Command mode

share/functions/fish_default_key_bindings.fish

@@ -1,18 +1,18 @@
 
 function fish_default_key_bindings -d "Default (Emacs-like) key bindings for fish"
 	if not set -q argv[1]
+		# Clear earlier bindings, if any
+		bind --erase --all
 		if test "$fish_key_bindings" != "fish_default_key_bindings"
 			# Allow the user to set the variable universally
 			set -q fish_key_bindings; or set -g fish_key_bindings
 			set fish_key_bindings fish_default_key_bindings # This triggers the handler, which calls us again and ensures the user_key_bindings are executed
 			return
 		end
-		# Clear earlier bindings, if any
-		bind --erase --all
 	end
 
     # These are shell-specific bindings that we share with vi mode.
-    __fish_shared_key_bindings
+    __fish_shared_key_bindings $argv
 
 	# This is the default binding, i.e. the one used if no other binding matches
 	bind $argv "" self-insert

share/functions/fish_vi_key_bindings.fish

@@ -1,6 +1,8 @@
 function fish_vi_key_bindings --description 'vi-like key bindings for fish'
     # Allow any argument to skip setting the variable.
     if not set -q argv[1]
+        # Only erase the bindings if called without argument to allow hybrid bindings.
+        bind --erase --all
         # Allow just calling this function to correctly set the bindings.
         # Because it's a rather discoverable name, users will execute it
         # and without this would then have subtly broken bindings.
@@ -22,13 +24,13 @@ function fish_vi_key_bindings --description 'vi-like key bindings for fish'
     # not end/home, we share those.
     set -l eol_keys \$ g\$
     set -l bol_keys \^ 0 g\^
-    if set -q argv[1]
+    # Ignore any argument that is not a valid mode name.
+    if set -q argv[1]; and contains -- $argv[1] insert default visual
         set init_mode $argv[1]
     end
 
     # Inherit shared key bindings.
     # Do this first so vi-bindings win over default.
-    bind --erase --all
     for mode in insert default visual
         __fish_shared_key_bindings -M $mode
     end