Factor out writing of commit for notedb updates

For the sake of rebuilding the notedb, we would like to write all
commits for all AbstractChangeUpdates on a project to the object
database, before committing a single BatchRefUpdate to update all the
refs.  This refactoring makes that possible.

Change-Id: I864b2845b32426a55a3f62910ae09910bc25901e
3 files changed