Implemented manual packfile switching in fast-import.

Implemented manual packfile switching in fast-import.

To help importers which are dealing with massive amounts of data
fast-import needs to be able to close the packfile it is currently
writing to and open a new packfile for any additional data that
will be received.  A new 'checkpoint' command has been introduced
which can be used by the frontend import process to force this
to occur at any time.  This may be useful to ensure a very long
running import doesn't lose any work due to unexpected failures.

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