Implemented automatic checkpoints within fast-import.

Implemented automatic checkpoints within fast-import.

When the number of objects or number of bytes gets close to the limit
allowed by the packfile format (or configured on the command line by
our caller) we should automatically checkpoint the current packfile
and start a new one before writing the object out.  This does however
require that we abandon the delta (if we had one) as its not valid
in a new packfile.

I also added the simple rule that if we got a delta back but the
delta itself is the same size as or larger than the uncompressed
object to ignore the delta and just store the object data.  This
should avoid some really bad behavior caused by our current delta
strategy.

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