Correct packfile edge output in fast-import.

Correct packfile edge output in fast-import.

Branches are only contained by a packfile if the branch actually
had its most recent commit in that packfile.  So new branches are
set to MAX_PACK_ID to ensure they don't cause their commit to list
as part of the first packfile when it closes out if the commit was
actually in existance before fast-import started.

Also corrected the type of last_commit to be umaxint_t to prevent
overflow and wraparound on very large imports.  Though that is
highly unlikely to occur as we're talking 4 billion commits, which
no real project has right now.

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