add -i: revisit hunk on editor failure

Similar to the behaviour for editing a commit message, let terminating
the editor with a failure abort the current hunk edit and revisit the
option selection for the hunk.

Signed-off-by: Deskin Miller <deskinm@umich.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-add--interactive.perl

@@ -753,6 +753,10 @@ EOF
 		|| $ENV{VISUAL} || $ENV{EDITOR} || "vi";
 	system('sh', '-c', $editor.' "$@"', $editor, $hunkfile);
 
+	if ($? != 0) {
+		return undef;
+	}
+
 	open $fh, '<', $hunkfile
 		or die "failed to open hunk edit file for reading: " . $!;
 	my @newtext = grep { !/^#/ } <$fh>;