Corrected buffer overflow during automatic checkpoint in fast-import.

Corrected buffer overflow during automatic checkpoint in fast-import.

If we previously were using a delta but we needed to checkpoint the
current packfile and switch to a new packfile we need to throw away
the delta and compress the raw object by itself, as delta chains
cannot span non-thin packfiles.  Unfortunately the output buffer
in this case needs to grow, as the size of the compressed object
may be quite a bit larger than the size of the compressed delta.

I've also avoided recompressing the object if we are checkpointing
and we didn't use a delta.  In this case the output buffer is the
correct size and has already been populated with the right data,
we just need to close out the current packfile and open a new one.

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