Release ObjectInserter in merge() not mergeImpl()

Release ObjectInserter in merge() not mergeImpl()

By doing the release at the higher level class, we can ensure
the release occurs if the inserter was allocated, even if the
implementation forgets to do this.  Since the higher level class
is what allocated it, it makes sense to have it also do the release.

Change-Id: Id617b2db864c3208ed68cba4eda80e51612359ad
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>