Allow for backend optimization of git_odb_exists queries

In particular, for some backends it is more expensive to repeatedly do
existence queries that will fail than it is to write objects which will
only rarely be written twice.  This patch adds a "confirm_not_exists"
query to git_odb_exists, so that libgit2 can tell the backend that it is
only checking for existence in order to avoid duplicating content by an
unnecessary write.