Close the completions pager (if it's open) on ctrl+c

If you're using the old binding that only clears the commandline and
doesn't preserve its contents and start a new line, you can use

```fish
bind \cc "commandline -f cancel; commandline ''"
```

instead.

Closes #4298.

share/functions/__fish_cancel_commandline.fish

@@ -1,5 +1,8 @@
 # This is meant to be bound to something like \cC.
 function __fish_cancel_commandline
+    # Close the pager if it's open (#4298)
+    commandline -f cancel
+
     set -l cmd (commandline)
     if test -n "$cmd"
         commandline -C 1000000