[zsh] Keep current $BUFFER on ALT-C

Ideally, we could only use `print -sr` to update the command history.
However, the "cd" command by ALT-C is added to the history only after we
finalize the current command by pressing an additional enter key.

i.e. The cd command from ALT-C is not visible when you hit Up arrow. But
it appears once you hit enter key.

So when the current buffer is empty, we use `zle accept-line` so that
the command history is immediately updated.

Close #2200

shell/key-bindings.zsh

@@ -87,10 +87,15 @@ fzf-cd-widget() {
     zle redisplay
     return 0
   fi
-  BUFFER="cd ${(q)dir}"
-  unset dir # ensure this doesn't end up appearing in prompt expansion
-  zle accept-line
+  if [ -z "$BUFFER" ]; then
+    BUFFER="cd ${(q)dir}"
+    zle accept-line
+  else
+    print -sr "cd ${(q)dir}"
+    cd "$dir"
+  fi
   local ret=$?
+  unset dir # ensure this doesn't end up appearing in prompt expansion
   zle fzf-redraw-prompt
   return $ret
 }