Restructure fast-import to support creating multiple packfiles.

Restructure fast-import to support creating multiple packfiles.

Now that we are starting to see some really large projects (such
as KDE or a fork of FreeBSD) get imported into Git we're running
into the upper limit on packfile object count as well as overall
byte length.  The KDE and FreeBSD projects are both likely to
require more than 4 GiB to store their current history, which means
we really need multiple packfiles to handle their content.

This is a fairly simple restructuring of the internal code to help
us support creating multiple packfiles from within fast-import.
We are now adding a 5 digit incrementing suffix to the end of the
basename supplied to us by the caller, permitting up to 99,999
packs to be generated in a single fast-import run.

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