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>

git-add--interactive.perl

@@ -45,6 +45,9 @@ my ($diff_new_color) =
 my $normal_color = $repo->get_color("", "reset");
 
 my $use_readkey = 0;
+my $use_termcap = 0;
+my %term_escapes;
+
 sub ReadMode;
 sub ReadKey;
 if ($repo->config_bool("interactive.singlekey")) {
@@ -53,6 +56,14 @@ if ($repo->config_bool("interactive.singlekey")) {
 		Term::ReadKey->import;
 		$use_readkey = 1;
 	};
+	eval {
+		require Term::Cap;
+		my $termcap = Term::Cap->Tgetent;
+		foreach (values %$termcap) {
+			$term_escapes{$_} = 1 if /^\e/;
+		}
+		$use_termcap = 1;
+	};
 }
 
 sub colored {
@@ -1067,6 +1078,14 @@ sub prompt_single_character {
 		ReadMode 'cbreak';
 		my $key = ReadKey 0;
 		ReadMode 'restore';
+		if ($use_termcap and $key eq "\e") {
+			while (!defined $term_escapes{$key}) {
+				my $next = ReadKey 0.5;
+				last if (!defined $next);
+				$key .= $next;
+			}
+			$key =~ s/\e/^[/;
+		}
 		print "$key" if defined $key;
 		print "\n";
 		return $key;