git-repack: repo.usedeltabaseoffset

git-repack: repo.usedeltabaseoffset

When configuration variable `repack.UseDeltaBaseOffset` is set
for the repository, the command passes `--delta-base-offset`
option to `git-pack-objects`; this typically results in slightly
smaller packs, but the generated packs are incompatible with
versions of git older than (and including) v1.4.3.

We will make it default to true sometime in the future, but not
for a while.

Signed-off-by: Junio C Hamano <junkio@cox.net>