function for editing command line in ext editor

This implements a standard function and bindings for editing the command
line in an external editor. This feature has been requested multiple
times in the past year with various solutions cut and pasted into those
issues. This change combines the best aspects of those solutions.

Fixes #1215

doc_src/index.hdr.in

@@ -1001,6 +1001,10 @@ Some bindings are shared between emacs- and vi-mode because they aren't text edi
 
 - @key{Alt,w} prints a short description of the command under the cursor.
 
+- @key{Alt,e} edit the current command line in an external editor. The editor is chosen from the first available of the `$VISUAL` or `$EDITOR` variables.
+
+- @key{Alt,v} Same as @key{Alt,e}.
+
 \subsection emacs-mode Emacs mode commands
 
 - @key{Home} or @key{Control,A} moves the cursor to the beginning of the line.

share/functions/__fish_shared_key_bindings.fish

@@ -96,4 +96,8 @@ function __fish_shared_key_bindings -d "Bindings shared between emacs and vi mod
     # Make it easy to turn an unexecuted command into a comment in the shell history. Also,
     # remove the commenting chars so the command can be further edited then executed.
     bind $argv \e\# __fish_toggle_comment_commandline
+
+    # The [meta-e] and [meta-v] keystrokes invoke an external editor on the command buffer.
+    bind \ee edit_command_buffer
+    bind \ev edit_command_buffer
 end

share/functions/edit_command_buffer.fish

@@ -0,0 +1,44 @@
+function edit_command_buffer --description 'Edit the command buffer in an external editor'
+    set -l f (mktemp)
+    if set -q f[1]
+        mv $f $f.fish
+        set f $f.fish
+    else
+        # We should never execute this block but better to be paranoid.
+        if set -q TMPDIR
+            set f $TMPDIR/fish.(echo %self).fish
+        else
+            set f /tmp/fish.(echo %self).fish
+        end
+        touch $f
+        or return 1
+    end
+
+    # Edit the command line with the users preferred editor or vim or emacs.
+    commandline -b >$f
+    if set -q VISUAL
+        eval $VISUAL $f
+    else if set -q EDITOR
+        eval $EDITOR $f
+    else
+        echo
+        echo (_ 'External editor requested but $VISUAL or $EDITOR not set.')
+        echo (_ 'Please set VISUAL or EDITOR to your preferred editor.')
+        commandline -f repaint
+        command rm $f
+        return 1
+    end
+
+    # Here we're checking the exit status of the editor.
+    if test $status -eq 0 -a -s $f
+        # Set the command to the output of the edited command and move the cursor to the
+        # end of the edited command.
+        commandline -r (cat $f)
+        commandline -C 999999
+    else
+        echo
+        echo (_ "Ignoring the output of your editor since its exit status was non-zero")
+        echo (_ "or the file was empty")
+    end
+    command rm $f
+end