Teach mergers to use ObjectInserter.newReader()

Teach mergers to use ObjectInserter.newReader()

This allows the RecursiveMerger to iteratively create new merge bases
without necessarily flushing packs to storage in the DFS case;
flushing only need happen at the end of the whole merge process.

Since Merger's walk now depends on its inserter, we need to construct
an inserter at Merger construction time.  This should not be a
significant increase in overhead since unused inserters don't use any
resources (beyond a reference to the Repository).

We also must release and recreate the walk whenever setObjectInserter
is called, which can break usages where setObjectInserter is called in
the middle of stateful operations on the walk.  No usages of this
method within JGit currently do this; the inserter is only ever set
before any stateful walk operations happen.

Change-Id: I9682a6aa4a2c37dccef8e163f132ddb791d79103