add -p: only display help for active keys

If the user presses a key that add -p wasn't expecting then it prints
a list of key bindings. Although the prompt only lists the active
bindings the help was printed for all bindings.  Fix this by using the
list of keys in the prompt to filter the help. Note that the list of
keys was already passed to help_patch_cmd() by the caller so there is
no change needed to the call site.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
