FileRepository: Support extensions.refsBackendType = RefTree

FileRepository: Support extensions.refsBackendType = RefTree

This experimental code can be enabled in $GIT_DIR/config:

  [core]
    repositoryformatversion = 1

  [extensions]
    refsBackendType = RefTree

When these are set the repository will read references from the
RefTree rooted by the $GIT_DIR/refs/txn/committed reference.

Update debug-rebuild-ref-tree to rebuild refs/txn/committed only from
the bootstrap layer.  This avoids misuse by rebuilding using packed-refs
and $GIT_DIR/refs tree.

Change-Id: Icf600e4a36b2f7867822a7ab1f1617d73c710a4b