Reapply "Create util class for work tree updating in both filesystem and index."

Reapply "Create util class for work tree updating in both filesystem and index."

This reverts commit 5709317f71ccaf26eceaa896150f203879b634b8.

Add a bugfix for deletions in ResolveMergers instantiated with just an
ObjectInserter as argument.

Original change description:

Create util class for work tree updating in both filesystem and index.

This class intends to make future support in index updating easier.

This class currently extracts some logic from ResolveMerger. Logic
related to StreamSupplier was copied from ApplyCommand, which will be
integrated in a following change.

Co-authored-by: Nitzan Gur-Furman <nitzan@google.com>
Co-authored-by: Han-Wen Nienhuys <hanwen@google.com>
Change-Id: Ideaefd51789a382a8b499d1ca7ae0146d032f48b