Merge branch 'master' into sp/fast-import

Merge branch 'master' into sp/fast-import

I'm bringing master in early so that the OBJ_OFS_DELTA implementation
is available as part of the topic.  This way git-fast-import can
learn about this new slightly smaller and faster packfile format,
and can generate them directly rather than needing to have them be
repacked with git-pack-objects.

Due to the API changes in master during the period of development
of git-fast-import, a few minor tweaks to fast-import.c are needed
to produce a working merge.  I've done them here as part of the
merge to ensure bisection always works.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>