add--interactive: leave main loop on read error

The main hunk loop for add--interactive will loop if it does
not get a known input. This is a good thing if the user
typed some invalid input. However, if we have an
uncorrectable read error, we'll end up looping infinitely.
We can fix this by noticing read errors (i.e., <STDIN>
returns undef) and breaking out of the loop.

One easy way to trigger this is if you have an editor that
does not take over the terminal (e.g., one that spawns a
window in an existing process and waits), start the editor
with the hunk-edit command, and hit ^C to send SIGINT. The
editor process dies due to SIGINT, but the perl
add--interactive process does not (perl suspends SIGINT for
the duration of our system() call).

We return to the main loop, but further reads from stdin
don't work. The SIGINT _also_ killed our parent git process,
which orphans our process group, meaning that further reads
from the terminal will always fail. We loop infinitely,
getting EIO on each read.

Note that there are several other spots where we read from
stdin, too. However, in each of those cases, we do something
sane when the read returns undef (breaking out of the loop,
taking the input as "no", etc). They don't need similar
treatment.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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