add -p: fix checking of user input

add -p: fix checking of user input

When a file has been deleted the C version of add -p allows the user
to edit a hunk even though 'e' is not in the list of allowed
responses. (I think 'e' is disallowed because if the file is edited it
is no longer a deletion and we're not set up to rewrite the diff
header).

The invalid response was allowed because the test that determines
whether to display 'e' was not duplicated correctly in the code that
processes the user's choice. Fix this by using flags that are set when
constructing the prompt and checked when processing the user's choice
rather than repeating the check itself.

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