In the DFS implementation, flushing an inserter writes a new pack to
the storage system and is potentially very slow, but was the only way
to ensure previously-inserted objects were available. For some tasks,
like performing a series of three-way merges, the total size of all
inserted objects may be small enough to avoid flushing the in-memory
buffered data.
DfsOutputStream already provides a read method to read back from the
not-yet-flushed data, so use this to provide an ObjectReader in the
DFS case.
In the file-backed case, objects are written out loosely on the fly,
so the implementation can just return the existing WindowCursor.
Change-Id: I454fdfb88f4d215e31b7da2b2a069853b197b3dd
Dave Borowitz