Use getObjectsDatabase().getDirectory() to find objects

Use getObjectsDatabase().getDirectory() to find objects

Only the ObjectDirectory type of database knows where to find the
objects directory on the local filesystem, so defer to it whenever
we need to know where the objects reside.  Since this is the type
returned by FileRepository's getObjectDatabase() method, we mostly
don't have to do much other than use a slightly longer invocation.

Change-Id: Ie5f58132a6411b56c3acad73646ad169d78a0654
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>