A subsequent patch will perform the same packfile replacement that is
already done twice, so refactor it into its own function. Also, the same
subsequent patch will use, in another way, part of the packfile
replacement functionality, so extract those out too.
Signed-off-by: Jonathan Tan <jonathantanmy@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>