add-patch: response to unknown command

add-patch: response to unknown command

When the user gives an unknown command to the "add -p" prompt, the list
of accepted commands with their explanation is given.  This is the same
output they get when they say '?'.

However, the unknown command may be due to a user input error rather
than the user not knowing the valid command.

To reduce the likelihood of user confusion and error repetition, instead
of displaying the list of accepted commands, display a short error
message with the unknown command received, as feedback to the user.

Include a reminder about the current command '?' in the new message, to
guide the user if they want help.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>