In commit 07e7dbf0d (gc: default aggressive depth to 50, 2016-08-11),
the default aggressive depth of git-gc has been changed to 50. While
git-config(1) has been updated to represent the new default value,
git-gc(1) still mentions the old value. This patch fixes it.
Signed-off-by: Patrick Steinhardt <ps@pks.im> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>