We ensure that the base and tip variables are defined before they

are referenced. Static analysis (via PyCharm) determines that the

variables are now always assigned before referencing.

Ran unit tests.
Before applying this patch, running rbt diff in an SVN repo against

two changesets that add an empty file and delete an empty file would

cause rbt to crash with the message:

CRITICAL: local variable 'base' referenced before assignment.

After applying this patch, this problem no longer persists. The output

of rbt diff has the correct revision numbers in this case.