Fix redundant file backups

We'd observed a bug in which an unchanged file was nevertheless
being redundantly transmitted for backup on every backup pass.
The underlying issue turns out to have been the FileBackupHelper
base implementation's logic for diffing the prior-state file
set against the current state, in the case when there had been
deletions of prior files.  In addition, there was also a
parallel bug in which file checksums were not calculated
properly in some cases, leading to at least one additional
redundant backup of the file in question.

Bug 18694053

Change-Id: Ie0dec06486b5fef4624561737019569c85d6b2a0