Provide default shallowCommits getter and setter in ObjectDatabase

Provide default shallowCommits getter and setter in ObjectDatabase

I649db9ae679ec2606cf7c530b040f8b6b93eb81a added a default implementation
for getShallowCommits and setShallowCommits to DfsObjDatabase, for the
convenience of any implementers that define subclasses. But we forgot
that some implementers inherit from ObjectDatabase directly instead.
Move the default getter and setter to the base class so that such
callers do not need source changes to unbreak their build.

This also lets us update the api_filters to reflect that this is no
longer an API-breaking change.

Change-Id: I5dcca462eb306e511e57907b7d9264d51b3f3014