add -i: ignore terminal escape sequences

add -i: ignore terminal escape sequences

On the author's terminal, the up-arrow input sequence is ^[[A, and
thus fat-fingering an up-arrow into 'git checkout -p' is quite
dangerous: git-add--interactive.perl will ignore the ^[ and [
characters and happily treat A as "discard everything".

As a band-aid fix, use Term::Cap to get all terminal capabilities.
Then use the heuristic that any capability value that starts with ^[
(i.e., \e in perl) must be a key input sequence.  Finally, given an
input that starts with ^[, read more characters until we have read a
full escape sequence, then return that to the caller.  We use a
timeout of 0.5 seconds on the subsequent reads to avoid getting stuck
if the user actually input a lone ^[.

Since none of the currently recognized keys start with ^[, the net
result is that the sequence as a whole will be ignored and the help
displayed.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>