At present we do headers unfolding (see RFC822 3.1.1. LONG HEADER FIELDS) for
all fields except 'From' (always) and 'Subject' (when keep_subject is set)

Not unfolding 'From' is a bug -- see above-mentioned RFC link.
Signed-off-by: NKirill Smelkov <kirr@landau.phys.spbu.ru>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>