Added tree and commit writing to fast-import.

Added tree and commit writing to fast-import.

The tree of the current commit can be altered by file_change commands
before the commit gets written to the pack.  The file changes are
rather primitive as they simply allow removal of a tree entry or
setting/adding a tree entry.

Currently trees and commits aren't being deltafied when written to
the pack and branch reloading from the current pack doesn't work,
so at most 5 branches can be worked with at any one time.

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