The list_and_choose() helper is given a prompt and a list, asks the
user to make selection from the list, and then returns a list of
items chosen.  Even when it is given an empty list as the original
candidate set to choose from, it gave a prompt to the user, who can
only say "I am done choosing".

Return an empty result when the input is an empty list without
bothering the user.  The existing caller must already have a logic
to say "Nothing to do" or an equivalent when the returned list is
empty (i.e. the user chose to select nothing) if it is necessary, so
no change to the callers is necessary.

This fixes the case where "add untracked" is asked in "git add -i"
and there is no untracked files in the working tree.  We used to give
an empty list of files to choose from with a prompt, but with this
change, we no longer do.

Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>