Remove RepositoryTestCase from DirCacheCGitCompatabilityTest

This test doesn't actually depend upon the large data set we have
in the RepositoryTestCase, so drop that from the dependency and
use the more simple LocalDiskRepositoryTestCase instead.

Change-Id: I0fd4affe1dd5ec86e8c3253db42df11d3b612e36
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>