Change git-gc documentation to reflect gc.packrefs implementation.
56752391a8c0c591853b276e4fa0b45c34ced181 (Make "git gc" pack all
refs by default) changed the default of gc.packrefs to true, to
pack all refs by default in any repository. IOW, the users need
to disable it explicitly if they want to by setting the config
variable, since 1.5.3.
However, we forgot to update the documentation. This fixes it.
Signed-off-by: Florian La Roche <laroche@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>