GitRepository.diff: add 'text' option

For generating textual diffs. Useful for Pq - for example, the 'patch'
utility does not support git binary diffs.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
GitRepository.diff: prevent usage of external diff

External diff might break patch generation, for example.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
ComponentTestBase: use eq_() ok_() from nose.tools

For better assert messages.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Remove unused import
Add link to online docs