Correct object_count type and stat output in fast-import.

Correct object_count type and stat output in fast-import.

Since object_count is limited to 'unsigned long' (really an
unsigned 32 bit integer value) by the pack file format we may as
well use exactly that type here in fast-import for that counter.
An earlier change by me incorrectly made it uintmax_t.

But since object_count is a counter for the current packfile only,
we don't want to output its value at the end.  Instead we should
sum up the individual type counters and report that total, as that
will cover all of the packfiles.

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