tests: unmask hidden test and fix test_diff

test_diff was getting masked by the later test_diff definition.
Rename the clobbering test_diff to test_commit_diff so that the
original test_diff is executed.

Fix the test_diff expectations.

Signed-off-by: David Aguilar <davvid@gmail.com>
Merge pull request #34 from davvid/fix-tests

tests: unmask hidden test and fix test_diff