Clean up pack tests.

Clean up pack tests.

-Remove trailing whitespace.
-Fix long lines.
-Hard-code SHAs in their hexlified forms.
-Use '' consistently.
-Always write to temp directories instead of working directory.
-Various indentation and local var refactoring fixes.
-Moved TestHexToSha to test_objects.py, since those functions are from
 objects.py.
-While we're in there, also removed trailing whitespace in pack.py.

Change-Id: I19f133e3d867f31fa75e28142593f84807fbc746