Expose the ObjectInserter that created an ObjectReader

Expose the ObjectInserter that created an ObjectReader

We've found in Gerrit Code Review that it is common to pass around
both an ObjectReader (or more commonly a RevWalk wrapping one) and an
ObjectInserter. These code paths often assume that the ObjectReader
can read back any objects created by the ObjectInserter without
flushing. However, we previously had no way to enforce that constraint
programmatically, leading to hard-to-spot problems.

Provide a solution by exposing the ObjectInserter that created an
ObjectReader, when known. Callers can either continue passing both
objects and check:
  reader.getCreatedFromInserter() == inserter
or they can just pass around ObjectReader and extract the inserter
when it's needed (checking that it's not null at usage time).

Change-Id: Ibbf5d1968b506f6b47030ab1b046ffccb47352ea