The current text makes some users feel uneasy, worrying whether
'-a' could lead to corrupt repositories. Clarify that '-a'
may lead to performance issues only for dumb protocols.
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net> Helped-by: Stephen Boyd <bebarino@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>