Three new unit tests, t06XX files have been added.

	t0601-read: tests for loading index files from disk,
				for creating in-memory indexes and for accessing
				index entries.
	t0602-write: tests for writing index files back to disk
	t0603-sort: tests for properly sorting the entries array of an index

Two test indexes have been added in 'tests/resources/':

	test/resources/index: a sample index from a libgit2 repository

	test/resources/gitgit.index: a sample index from a git.git
		repository (includes TREE extension data)

Signed-off-by: Vicent Marti <tanoku@gmail.com>