chainlint.sed: make here-doc "<<-" operator recognition more POSIX-like

chainlint.sed: make here-doc "<<-" operator recognition more POSIX-like

According to POSIX, "<<" and "<<-" are distinct shell operators. For the
latter to be recognized, no whitespace is allowed before the "-", though
whitespace is allowed after the operator. However, the chainlint
patterns which identify here-docs are both too loose and too tight,
incorrectly allowing whitespace between "<<" and "-" but disallowing it
between "-" and the here-doc tag. Fix the patterns to better match
POSIX.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>